diff --git a/gocardless_pro/api_client.py b/gocardless_pro/api_client.py
index d888496c..27c52e11 100644
--- a/gocardless_pro/api_client.py
+++ b/gocardless_pro/api_client.py
@@ -126,11 +126,18 @@ def _handle_errors(self, response):
             response_body = response.json()
         except ValueError:
             msg = 'Malformed response received from server'
-            raise errors.MalformedResponseError(msg, response.text)
+            raise errors.MalformedResponseError(msg, response.text, response.status_code)
 
         if response.status_code < 400:
             return
 
+        if not isinstance(response_body, dict):
+            raise errors.MalformedResponseError(
+                'Malformed response received from server',
+                response.text,
+                response.status_code,
+            )
+
         error = response_body.get('error', response_body)
 
         if isinstance(error, str):
@@ -139,8 +146,14 @@ def _handle_errors(self, response):
                 'message': error,
             }
             exception_class = errors.ApiError
-        else:
+        elif isinstance(error, dict) and 'type' in error:
             exception_class = errors.ApiError.exception_for(response.status_code, error['type'], error.get('errors'))
+        else:
+            raise errors.MalformedResponseError(
+                'Malformed response received from server',
+                response.text,
+                response.status_code,
+            )
 
         raise exception_class(error)
 
diff --git a/gocardless_pro/errors.py b/gocardless_pro/errors.py
index 6688e374..f37ee6fc 100644
--- a/gocardless_pro/errors.py
+++ b/gocardless_pro/errors.py
@@ -137,9 +137,26 @@ class RateLimitError(ApiError):
 
 class MalformedResponseError(GoCardlessProError):
 
-    def __init__(self, message, response):
-        super(MalformedResponseError, self).__init__(message)
+    BODY_PREVIEW_MAX_LENGTH = 500
+
+    def __init__(self, message, response, status_code=None):
+        self.status_code = status_code
         self.response = response
+        super(MalformedResponseError, self).__init__(
+            self._build_message(message, status_code, response)
+        )
+
+    @classmethod
+    def _build_message(cls, message, status_code, response):
+        parts = [message]
+        if status_code is not None:
+            parts.append('(HTTP {})'.format(status_code))
+        full = ' '.join(parts)
+        if response:
+            preview = response if len(response) <= cls.BODY_PREVIEW_MAX_LENGTH \
+                else response[:cls.BODY_PREVIEW_MAX_LENGTH] + '...'
+            full = '{}: {}'.format(full, preview)
+        return full
 
 
 class InvalidSignatureError(GoCardlessProError):
diff --git a/gocardless_pro/services/billing_request_flows_service.py b/gocardless_pro/services/billing_request_flows_service.py
index 96fe3f65..804a9e03 100644
--- a/gocardless_pro/services/billing_request_flows_service.py
+++ b/gocardless_pro/services/billing_request_flows_service.py
@@ -58,7 +58,7 @@ def initialise(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/billing_requests_service.py b/gocardless_pro/services/billing_requests_service.py
index 75485e95..2e94bbc9 100644
--- a/gocardless_pro/services/billing_requests_service.py
+++ b/gocardless_pro/services/billing_requests_service.py
@@ -75,7 +75,7 @@ def collect_customer_details(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -126,7 +126,7 @@ def collect_bank_account(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -153,7 +153,7 @@ def confirm_payer_details(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -179,7 +179,7 @@ def fulfil(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -205,7 +205,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -284,7 +284,7 @@ def notify(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -309,7 +309,7 @@ def fallback(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -339,7 +339,7 @@ def choose_currency(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -363,7 +363,7 @@ def select_institution(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/blocks_service.py b/gocardless_pro/services/blocks_service.py
index b57e1a24..2b4cb044 100644
--- a/gocardless_pro/services/blocks_service.py
+++ b/gocardless_pro/services/blocks_service.py
@@ -113,7 +113,7 @@ def disable(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -138,7 +138,7 @@ def enable(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -162,7 +162,7 @@ def block_by_ref(self,params=None, headers=None):
         path = '/blocks/block_by_ref'
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/creditor_bank_accounts_service.py b/gocardless_pro/services/creditor_bank_accounts_service.py
index b9ca1893..9237e510 100644
--- a/gocardless_pro/services/creditor_bank_accounts_service.py
+++ b/gocardless_pro/services/creditor_bank_accounts_service.py
@@ -120,7 +120,7 @@ def disable(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/customer_bank_accounts_service.py b/gocardless_pro/services/customer_bank_accounts_service.py
index ccc0c9f0..2dbec878 100644
--- a/gocardless_pro/services/customer_bank_accounts_service.py
+++ b/gocardless_pro/services/customer_bank_accounts_service.py
@@ -157,7 +157,7 @@ def disable(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/customer_notifications_service.py b/gocardless_pro/services/customer_notifications_service.py
index 45b92e20..3e59af4d 100644
--- a/gocardless_pro/services/customer_notifications_service.py
+++ b/gocardless_pro/services/customer_notifications_service.py
@@ -43,7 +43,7 @@ def handle(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/funds_availabilities_service.py b/gocardless_pro/services/funds_availabilities_service.py
index c8b80b96..72af7fdc 100644
--- a/gocardless_pro/services/funds_availabilities_service.py
+++ b/gocardless_pro/services/funds_availabilities_service.py
@@ -14,7 +14,7 @@ class FundsAvailabilitiesService(base_service.BaseService):
     """
 
     RESOURCE_CLASS = resources.FundsAvailability
-    RESOURCE_NAME = 'funds_availabilities'
+    RESOURCE_NAME = 'funds_availability'
 
 
     def check(self,identity,params=None, headers=None):
diff --git a/gocardless_pro/services/instalment_schedules_service.py b/gocardless_pro/services/instalment_schedules_service.py
index a24fc662..98bd28d0 100644
--- a/gocardless_pro/services/instalment_schedules_service.py
+++ b/gocardless_pro/services/instalment_schedules_service.py
@@ -205,7 +205,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/mandate_imports_service.py b/gocardless_pro/services/mandate_imports_service.py
index 748cb113..58b863de 100644
--- a/gocardless_pro/services/mandate_imports_service.py
+++ b/gocardless_pro/services/mandate_imports_service.py
@@ -103,7 +103,7 @@ def submit(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -133,7 +133,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/mandates_service.py b/gocardless_pro/services/mandates_service.py
index a7d5dcc8..30bb173d 100644
--- a/gocardless_pro/services/mandates_service.py
+++ b/gocardless_pro/services/mandates_service.py
@@ -143,7 +143,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -178,7 +178,7 @@ def reinstate(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/outbound_payments_service.py b/gocardless_pro/services/outbound_payments_service.py
index db35614d..1ecaa82e 100644
--- a/gocardless_pro/services/outbound_payments_service.py
+++ b/gocardless_pro/services/outbound_payments_service.py
@@ -60,7 +60,7 @@ def withdraw(self,params=None, headers=None):
         path = '/outbound_payments/withdrawal'
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -88,7 +88,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -113,7 +113,7 @@ def approve(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/payer_authorisations_service.py b/gocardless_pro/services/payer_authorisations_service.py
index ec7beddb..d99f0988 100644
--- a/gocardless_pro/services/payer_authorisations_service.py
+++ b/gocardless_pro/services/payer_authorisations_service.py
@@ -129,7 +129,7 @@ def submit(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -164,7 +164,7 @@ def confirm(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/payments_service.py b/gocardless_pro/services/payments_service.py
index 3a736ee0..c84c69ab 100644
--- a/gocardless_pro/services/payments_service.py
+++ b/gocardless_pro/services/payments_service.py
@@ -149,7 +149,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -182,7 +182,7 @@ def retry(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/redirect_flows_service.py b/gocardless_pro/services/redirect_flows_service.py
index 7aeb4e36..73f98d66 100644
--- a/gocardless_pro/services/redirect_flows_service.py
+++ b/gocardless_pro/services/redirect_flows_service.py
@@ -97,7 +97,7 @@ def complete(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/scenario_simulators_service.py b/gocardless_pro/services/scenario_simulators_service.py
index 2b9ddebc..6f40aa21 100644
--- a/gocardless_pro/services/scenario_simulators_service.py
+++ b/gocardless_pro/services/scenario_simulators_service.py
@@ -65,7 +65,7 @@ def run(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/subscriptions_service.py b/gocardless_pro/services/subscriptions_service.py
index be5d3c84..dff2cc57 100644
--- a/gocardless_pro/services/subscriptions_service.py
+++ b/gocardless_pro/services/subscriptions_service.py
@@ -206,7 +206,7 @@ def pause(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -246,7 +246,7 @@ def resume(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
@@ -275,7 +275,7 @@ def cancel(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/gocardless_pro/services/webhooks_service.py b/gocardless_pro/services/webhooks_service.py
index 9a12982a..1fed0f12 100644
--- a/gocardless_pro/services/webhooks_service.py
+++ b/gocardless_pro/services/webhooks_service.py
@@ -85,7 +85,7 @@ def retry(self,identity,params=None, headers=None):
           })
         
         if params is not None:
-            params = {'data': params}
+            params = {self._envelope_key(): params}
         response = self._perform_request('POST', path, params, headers,
                                          retry_failures=False)
         return self._resource_for(response)
diff --git a/tests/api_client_test.py b/tests/api_client_test.py
index 6da7044b..53509d3e 100644
--- a/tests/api_client_test.py
+++ b/tests/api_client_test.py
@@ -125,6 +125,19 @@ def test_handles_malformed_response():
     with pytest.raises(errors.MalformedResponseError):
         client.post('/test', body={'name': 'Billy Jean'})
 
+@responses.activate
+def test_includes_status_code_and_body_for_non_json_error_response():
+    body = '<html>502 Bad Gateway</html>'
+    responses.add(responses.POST, 'http://example.com/test',
+                  body=body, status=502)
+
+    with pytest.raises(errors.MalformedResponseError) as exception:
+        client.post('/test', body={'name': 'Billy Jean'})
+
+    assert exception.value.status_code == 502
+    assert exception.value.response == body
+    assert 'HTTP 502' in str(exception.value)
+
 @responses.activate
 def test_handles_valid_empty_response():
     responses.add(responses.DELETE, 'http://example.com/test', body='', status=204)