diff -r 48228fb874c1 Modules/zlibmodule.c
--- a/Modules/zlibmodule.c	Mon Nov 05 09:34:46 2012 +0200
+++ b/Modules/zlibmodule.c	Mon Nov 05 13:13:00 2012 +0200
@@ -972,6 +972,8 @@
     ENTER_ZLIB(self);
 
     start_total_out = self->zst.total_out;
+    self->zst.avail_in = PyBytes_GET_SIZE(self->unconsumed_tail);
+    self->zst.next_in = (Byte *)PyBytes_AS_STRING(self->unconsumed_tail);
     self->zst.avail_out = length;
     self->zst.next_out = (Byte *)PyBytes_AS_STRING(retval);