diff --git a/libavcodec/mpeg4videodec.c b/libavcodec/mpeg4videodec.c
index 673c4e865c..617dcb9f18 100644
--- a/libavcodec/mpeg4videodec.c
+++ b/libavcodec/mpeg4videodec.c
@@ -1494,16 +1494,17 @@ end:
 
 static int mpeg4_decode_gop_header(MpegEncContext * s, GetBitContext *gb){
     int hours, minutes, seconds;
+    unsigned time_code = show_bits(gb, 18);
 
-    hours= get_bits(gb, 5);
-    minutes= get_bits(gb, 6);
-    skip_bits1(gb);
-    seconds= get_bits(gb, 6);
-
-    s->time_base= seconds + 60*(minutes + 60*hours);
-
-    skip_bits1(gb);
-    skip_bits1(gb);
+    if (time_code & 0x40) {     /* marker_bit */
+        hours   = time_code >> 13;
+        minutes = time_code >>  7 & 0x3f;
+        seconds = time_code       & 0x3f;
+        s->time_base = seconds + 60*(minutes + 60*hours);
+        skip_bits(gb, 20);      /* time_code, closed_gov, broken_link */
+    } else {
+        av_log(s->avctx, AV_LOG_WARNING, "GOP header missing marker_bit\n");
+    }
 
     return 0;
 }