diff -r feed6d2097b1 Modules/itertoolsmodule.c
--- a/Modules/itertoolsmodule.c Wed Sep 14 15:19:42 2011 +0200
+++ b/Modules/itertoolsmodule.c Thu Sep 15 18:52:59 2011 +0200
@@ -1234,7 +1234,7 @@
         return NULL;
     lz->cnt++;
     oldnext = lz->next;
-    lz->next += lz->step;
+    lz->next = (size_t)lz->next + lz->step;
     if (lz->next < oldnext || (stop != -1 && lz->next > stop))
         lz->next = stop;
     return item;