diff --git a/qemu-img.c b/qemu-img.c
index 0db8d4f194..0465e4a3a8 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1224,7 +1224,7 @@ static int img_rebase(int argc, char **argv)
                 int pnum;
 
                 if (compare_sectors(buf_old + written * 512,
-                    buf_new + written * 512, n, &pnum))
+                    buf_new + written * 512, n - written, &pnum))
                 {
                     ret = bdrv_write(bs, sector + written,
                         buf_old + written * 512, pnum);