diff --git a/block/dmg.c b/block/dmg.c
index 2c806e3389a385b1ffe4973ae687c7d0b6d93391..b26ddb1f68520a3662e20f91759d6fa7ad472860 100644
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -572,16 +572,20 @@ static inline uint32_t search_chunk(BDRVDMGState *s, uint64_t sector_num)
 {
     /* binary search */
     uint32_t chunk1 = 0, chunk2 = s->n_chunks, chunk3;
-    while (chunk1 != chunk2) {
+    while (chunk1 <= chunk2) {
         chunk3 = (chunk1 + chunk2) / 2;
         if (s->sectors[chunk3] > sector_num) {
-            chunk2 = chunk3;
+            if (chunk3 == 0) {
+                goto err;
+            }
+            chunk2 = chunk3 - 1;
         } else if (s->sectors[chunk3] + s->sectorcounts[chunk3] > sector_num) {
             return chunk3;
         } else {
-            chunk1 = chunk3;
+            chunk1 = chunk3 + 1;
         }
     }
+err:
     return s->n_chunks; /* error */
 }