diff -r 7cfbebadb90b Lib/socket.py
--- a/Lib/socket.py	Tue Feb 11 08:13:10 2014 -0500
+++ b/Lib/socket.py	Tue Feb 11 22:35:39 2014 +0100
@@ -200,7 +200,7 @@
         """
         for c in mode:
             if c not in {"r", "w", "b"}:
-                raise ValueError("invalid mode %r (only r, w, b allowed)")
+                raise ValueError("invalid mode %r (only r, w, b allowed)" % c)
         writing = "w" in mode
         reading = "r" in mode or not writing
         assert reading or writing