diff --git a/mercurial/bdiff.c b/mercurial/bdiff.c
--- a/mercurial/bdiff.c
+++ b/mercurial/bdiff.c
@@ -255,8 +255,8 @@ static struct hunklist diff(struct line 
 	if (pos && l.base && t) {
 		/* generate the matching block list */
 		recurse(a, b, pos, 0, an, 0, bn, &l);
-		l.head->a1 = an;
-		l.head->b1 = bn;
+		l.head->a1 = l.head->a2 = an;
+		l.head->b1 = l.head->b2 = bn;
 		l.head++;
 	}