diff --git a/pkg/replica/diff_disk.go b/pkg/replica/diff_disk.go
index 64405ebff..69aae608a 100644
--- a/pkg/replica/diff_disk.go
+++ b/pkg/replica/diff_disk.go
@@ -237,7 +237,7 @@ func (d *diffDisk) read(target byte, buf []byte, startOffset int64, startSector
 
 	// May read the expanded part
 	if offset >= size {
-		return 0, nil
+		return int(bufLength), nil
 	}
 	var newBuf []byte
 	if bufLength > size-offset {