diff --git a/fastapi/openapi/utils.py b/fastapi/openapi/utils.py
index 7efbe796eb..dbf0cf595c 100644
--- a/fastapi/openapi/utils.py
+++ b/fastapi/openapi/utils.py
@@ -345,7 +345,7 @@ def get_openapi_path(
                         *route.return_response_classes,
                     )
                 else:
-                    response_classes = route.return_response_classes
+                    response_classes = tuple(route.return_response_classes)
             else:
                 response_classes = (current_response_class,)