diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 6cbc02aa0f..7633ef6342 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -391,9 +391,10 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
     uint64_t num_dirty = 0;
     unsigned long *dest = rb->bmap;
 
-    /* start address is aligned at the start of a word? */
+    /* start address and length is aligned at the start of a word? */
     if (((word * BITS_PER_LONG) << TARGET_PAGE_BITS) ==
-         (start + rb->offset)) {
+         (start + rb->offset) &&
+        !(length & ((BITS_PER_LONG << TARGET_PAGE_BITS) - 1))) {
         int k;
         int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
         unsigned long * const *src;