--- a/mercurial/bdiff.c
+++ b/mercurial/bdiff.c
@@ -239,7 +239,7 @@ static struct hunklist diff(struct line 
 
 static PyObject *blocks(PyObject *self, PyObject *args)
 {
-	PyObject *sa, *sb, *rl, *m;
+	PyObject *sa, *sb, *rl = NULL, *m;
 	struct line *a, *b;
 	struct hunklist l;
 	struct hunk *h;