visit_decref():  Added another assert.
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index a623e8b..6df2485 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -224,6 +224,7 @@
 		 * generation being collected, which can be recognized
 		 * because only they have positive gc_refs.
 		 */
+		assert(gc->gc.gc_refs != 0); /* else refcount was too small */
 		if (gc->gc.gc_refs > 0)
 			gc->gc.gc_refs--;
 	}