diff -r 0a9143d7b097 Lib/zipfile.py
--- a/Lib/zipfile.py	Thu May 03 13:43:07 2012 +0200
+++ b/Lib/zipfile.py	Sun May 13 20:31:12 2012 +0200
@@ -656,12 +656,13 @@
                 buf += self._read1(self.MAX_N)
             return buf
 
-        n -= len(self._readbuffer) - self._offset
-        if n < 0:
+        n += self._offset
+        if n < len(self._readbuffer):
             buf = self._readbuffer[self._offset:n]
-            self._offset += len(buf)
+            self._offset = n
             return buf
 
+        n -= len(self._readbuffer)
         buf = self._readbuffer[self._offset:]
         self._readbuffer = b''
         self._offset = 0
@@ -697,12 +698,13 @@
             buf += data
             return buf
 
-        n -= len(self._readbuffer) - self._offset
-        if n < 0:
+        n += self._offset
+        if n < len(self._readbuffer):
             buf = self._readbuffer[self._offset:n]
-            self._offset += len(buf)
+            self._offset = n
             return buf
 
+        n -= len(self._readbuffer)
         buf = self._readbuffer[self._offset:]
         self._readbuffer = b''
         self._offset = 0