diff --git a/src/google/appengine/runtime/default_api_stub.py b/src/google/appengine/runtime/default_api_stub.py
index d26399c..8294949 100755
--- a/src/google/appengine/runtime/default_api_stub.py
+++ b/src/google/appengine/runtime/default_api_stub.py
@@ -72,9 +72,12 @@
 POOL_SIZE_UPPER_LIMIT = 100
 POOL_SIZE_DEFAULT_VALUE = 10
 
-URLLIB3_POOL_SIZE = int(
-    os.environ.get('POOL_SIZE_URLLIB3', POOL_SIZE_DEFAULT_VALUE)
-)
+try:
+  URLLIB3_POOL_SIZE = int(
+      os.environ.get('POOL_SIZE_URLLIB3', POOL_SIZE_DEFAULT_VALUE)
+  )
+except (ValueError, TypeError):
+  URLLIB3_POOL_SIZE = POOL_SIZE_DEFAULT_VALUE
 
 if not (POOL_SIZE_LOWER_LIMIT <= URLLIB3_POOL_SIZE <= POOL_SIZE_UPPER_LIMIT):
   URLLIB3_POOL_SIZE = POOL_SIZE_DEFAULT_VALUE