diff -up Python-2.7.3/Lib/StringIO.py.old Python-2.7.3/Lib/StringIO.py
--- Python-2.7.3/Lib/StringIO.py.old	2013-10-10 11:47:18.165660213 +0200
+++ Python-2.7.3/Lib/StringIO.py	2013-10-10 11:49:00.981343754 +0200
@@ -158,7 +158,7 @@ class StringIO:
             newpos = self.len
         else:
             newpos = i+1
-        if length is not None and length > 0:
+        if length >= 0:
             if self.pos + length < newpos:
                 newpos = self.pos + length
         r = self.buf[self.pos:newpos]