diff --git a/twitter.py b/twitter.py
index 482dc44c..63d0a242 100644
--- a/twitter.py
+++ b/twitter.py
@@ -4717,7 +4717,7 @@ def GetListMembers(self, list_id, slug, owner_id=None, owner_screen_name=None, c
     if list_id is None:
       if slug is None:
         raise TwitterError('list_id or slug required')
-      if owner_id is None and owner_screen_name:
+      if owner_id is None and not owner_screen_name:
         raise TwitterError('if list_id is not given you have to include an owner to help identify the proper list')
 
     if owner_id: