diff --git a/libavcodec/mpeg12.c b/libavcodec/mpeg12.c
index 906643de16..b7af9262d7 100644
--- a/libavcodec/mpeg12.c
+++ b/libavcodec/mpeg12.c
@@ -948,7 +948,7 @@ static int mpeg1_decode_block(MpegEncContext *s,
         block[j] = level;
         i++;
     }
-    s->block_last_index[n] = i;
+    s->block_last_index[n] = i-1;
     return 0;
 }