diff --git a/libavcodec/vc1dec.c b/libavcodec/vc1dec.c
index bdab48ca7f..fa25161d75 100644
--- a/libavcodec/vc1dec.c
+++ b/libavcodec/vc1dec.c
@@ -1149,8 +1149,12 @@ static av_always_inline void get_mvdata_interlaced(VC1Context *v, int *dmv_x,
         *dmv_x = get_bits(gb, v->k_x);
         *dmv_y = get_bits(gb, v->k_y);
         if (v->numref) {
-            *pred_flag = *dmv_y & 1;
-            *dmv_y     = (*dmv_y + *pred_flag) >> 1;
+            if (pred_flag) {
+                *pred_flag = *dmv_y & 1;
+                *dmv_y     = (*dmv_y + *pred_flag) >> 1;
+            } else {
+                *dmv_y     = (*dmv_y + (*dmv_y & 1)) >> 1;
+            }
         }
     }
     else {
@@ -1177,7 +1181,7 @@ static av_always_inline void get_mvdata_interlaced(VC1Context *v, int *dmv_x,
             *dmv_y = (sign ^ ((val >> 1) + offs_tab[index1 >> v->numref])) - sign;
         } else
             *dmv_y = 0;
-        if (v->numref)
+        if (v->numref && pred_flag)
             *pred_flag = index1 & 1;
     }
 }
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 4da4f1851f..87c4a39d0e 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1881,6 +1881,7 @@ static int matroska_deliver_packet(MatroskaDemuxContext *matroska,
  */
 static void matroska_clear_queue(MatroskaDemuxContext *matroska)
 {
+    matroska->prev_pkt = NULL;
     if (matroska->packets) {
         int n;
         for (n = 0; n < matroska->num_packets; n++) {
@@ -2388,7 +2389,6 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
         avio_seek(s->pb, st->index_entries[st->nb_index_entries-1].pos, SEEK_SET);
         matroska->current_id = 0;
         while ((index = av_index_search_timestamp(st, timestamp, flags)) < 0) {
-            matroska->prev_pkt = NULL;
             matroska_clear_queue(matroska);
             if (matroska_parse_cluster(matroska) < 0)
                 break;