diff --git a/libavcodec/h261dec.c b/libavcodec/h261dec.c
index 7054286..aa1be6f 100644
--- a/libavcodec/h261dec.c
+++ b/libavcodec/h261dec.c
@@ -285,6 +285,10 @@
 
     // Read mtype
     h->mtype = get_vlc2(&s->gb, h261_mtype_vlc.table, H261_MTYPE_VLC_BITS, 2);
+    if (h->mtype < 0) {
+        av_log(s->avctx, AV_LOG_ERROR, "illegal mtype %d\n", h->mtype);
+        return SLICE_ERROR;
+    }
     h->mtype = h261_mtype_map[h->mtype];
 
     // Read mquant