diff --git a/google/auth/_default.py b/google/auth/_default.py
index 63009dfb8..13c03dc3d 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -250,23 +250,23 @@ def _get_explicit_environ_credentials(quota_project_id=None):
     from google.auth import _cloud_sdk
 
     cloud_sdk_adc_path = _cloud_sdk.get_application_default_credentials_path()
-    explicit_file = os.environ.get(environment_vars.CREDENTIALS)
+    explicit_file = os.environ.get(environment_vars.CREDENTIALS, "")
 
     _LOGGER.debug(
-        "Checking %s for explicit credentials as part of auth process...", explicit_file
+        "Checking '%s' for explicit credentials as part of auth process...", explicit_file
     )
 
-    if explicit_file is not None and explicit_file == cloud_sdk_adc_path:
+    if explicit_file != "" and explicit_file == cloud_sdk_adc_path:
         # Cloud sdk flow calls gcloud to fetch project id, so if the explicit
         # file path is cloud sdk credentials path, then we should fall back
         # to cloud sdk flow, otherwise project id cannot be obtained.
         _LOGGER.debug(
-            "Explicit credentials path %s is the same as Cloud SDK credentials path, fall back to Cloud SDK credentials flow...",
+            "Explicit credentials path '%s' is the same as Cloud SDK credentials path, fall back to Cloud SDK credentials flow...",
             explicit_file,
         )
         return _get_gcloud_sdk_credentials(quota_project_id=quota_project_id)
 
-    if explicit_file is not None:
+    if explicit_file != "":
         credentials, project_id = load_credentials_from_file(
             os.environ[environment_vars.CREDENTIALS], quota_project_id=quota_project_id
         )