diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 89df095cd8..237e9295e0 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1783,7 +1783,7 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
                         lace_size[n] = lace_size[n - 1] + snum;
                         total += lace_size[n];
                     }
-                    lace_size[n] = size - total;
+                    lace_size[laces - 1] = size - total;
                     break;
                 }
             }