diff --git a/src/urllib3/connectionpool.py b/src/urllib3/connectionpool.py
index e275a7dbad..7799b9969e 100644
--- a/src/urllib3/connectionpool.py
+++ b/src/urllib3/connectionpool.py
@@ -1894,7 +1894,8 @@ def urlopen(
             assert isinstance(response, ResponsePromise)
             return response  # actually a response promise!
 
-        assert isinstance(response, HTTPResponse)
+        if not isinstance(response, HTTPResponse):
+            warnings.warn(f"Expected {type(response)} to be a subclass of HTTPResponse")
 
         if redirect and response.get_redirect_location():
             # Handle redirect?
diff --git a/src/urllib3/poolmanager.py b/src/urllib3/poolmanager.py
index f994287db9..306662bf65 100644
--- a/src/urllib3/poolmanager.py
+++ b/src/urllib3/poolmanager.py
@@ -914,7 +914,9 @@ def urlopen(
             # the established connection is not capable of doing multiplexed request
             kw["multiplexed"] = False
 
-        assert isinstance(response, HTTPResponse)
+        if not isinstance(response, HTTPResponse):
+            warnings.warn(f"Expected {type(response)} to be a subclass of HTTPResponse")
+
         redirect_location = redirect and response.get_redirect_location()
         if not redirect_location:
             return response