Index: Lib/gzip.py
===================================================================
--- Lib/gzip.py	(revision 86421)
+++ Lib/gzip.py	(working copy)
@@ -91,7 +91,7 @@
             if hasattr(fileobj, 'name'): filename = fileobj.name
             else: filename = ''
         if mode is None:
-            if hasattr(fileobj, 'mode'): mode = fileobj.mode
+            if hasattr(fileobj, 'mode') and fileobj.mode is not None: mode = fileobj.mode
             else: mode = 'rb'
 
         if mode[0:1] == 'r':