diff --git a/src/sna/sna.h b/src/sna/sna.h
index ff1c9c11..b5bebe80 100644
--- a/src/sna/sna.h
+++ b/src/sna/sna.h
@@ -1208,11 +1208,13 @@ find_clip_box_for_y(const BoxRec *begin, const BoxRec *end, int16_t y)
 
 	/* Quick test if scanline is within range of clip boxes */
 	if (begin->y2 > y) {
-		assert(__find_clip_box_for_y(begin, end, y) == begin);
+		assert(end == begin + 1 ||
+		       __find_clip_box_for_y(begin, end, y) == begin);
 		return begin;
 	}
 	if (y >= end[-1].y2) {
-		assert(__find_clip_box_for_y(begin, end, y) == end);
+		assert(end == begin + 1 ||
+		       __find_clip_box_for_y(begin, end, y) == end);
 		return end;
 	}
 
diff --git a/src/sna/sna_accel.c b/src/sna/sna_accel.c
index 29bf6c4d..467d95f9 100644
--- a/src/sna/sna_accel.c
+++ b/src/sna/sna_accel.c
@@ -7165,14 +7165,14 @@ sna_copy_area(DrawablePtr src, DrawablePtr dst, GCPtr gc,
 const BoxRec *
 __find_clip_box_for_y(const BoxRec *begin, const BoxRec *end, int16_t y)
 {
-	assert(begin != end);
-	while (end > begin + 1) {
+	assert(end - begin > 1);
+	do {
 		const BoxRec *mid = begin + (end - begin) / 2;
 		if (mid->y2 > y)
 			end = mid;
 		else
 			begin = mid;
-	}
+	} while (end > begin + 1);
 	if (begin->y2 > y)
 		return begin;
 	else