--- stackless/Modules/zlibmodule.c	2010-12-08 21:40:48.000000000 0000
+++ stackless\Modules\zlibmodule.c	2010-12-08 21:40:48.000000000 0000
@@ -73,7 +73,10 @@
 zlib_error(z_stream zst, int err, char *msg)
 {
     const char *zmsg = zst.msg;
-    if (zmsg == Z_NULL) {
+	// Check for version error before using msg, as msg is not set in this case.
+	if (err == Z_VERSION_ERROR) {
+		zmsg = "different compile and link zlib versions";
+	} else if (zmsg == Z_NULL) {
         switch (err) {
         case Z_BUF_ERROR:
             zmsg = "incomplete or truncated stream";