diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index cd7ece8458..087b0b45e1 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -1436,6 +1436,8 @@ int av_write_frame(AVFormatContext *s, AVPacket *pkt);
  * @param s media file handle
  * @param pkt The packet containing the data to be written. Libavformat takes
  * ownership of the data and will free it when it sees fit using the packet's
+ * This can be NULL (at any time, not just at the end), to flush the
+ * interleaving queues.
  * @ref AVPacket.destruct "destruct" field. The caller must not access the data
  * after this function returns, as it may already be freed.
  * Packet's @ref AVPacket.stream_index "stream_index" field must be set to the
diff --git a/libavformat/utils.c b/libavformat/utils.c
index 6c985af1a0..cb708addf4 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -3128,24 +3128,30 @@ static int interleave_packet(AVFormatContext *s, AVPacket *out, AVPacket *in, in
 }
 
 int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt){
-    AVStream *st= s->streams[ pkt->stream_index];
-    int ret;
+    int ret, flush = 0;
 
-    //FIXME/XXX/HACK drop zero sized packets
-    if(st->codec->codec_type == AVMEDIA_TYPE_AUDIO && pkt->size==0)
-        return 0;
+    if (pkt) {
+        AVStream *st= s->streams[ pkt->stream_index];
 
-    av_dlog(s, "av_interleaved_write_frame size:%d dts:%"PRId64" pts:%"PRId64"\n",
-            pkt->size, pkt->dts, pkt->pts);
-    if((ret = compute_pkt_fields2(s, st, pkt)) < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
-        return ret;
+        //FIXME/XXX/HACK drop zero sized packets
+        if(st->codec->codec_type == AVMEDIA_TYPE_AUDIO && pkt->size==0)
+            return 0;
 
-    if(pkt->dts == AV_NOPTS_VALUE && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
-        return AVERROR(EINVAL);
+        av_dlog(s, "av_interleaved_write_frame size:%d dts:%"PRId64" pts:%"PRId64"\n",
+                pkt->size, pkt->dts, pkt->pts);
+        if((ret = compute_pkt_fields2(s, st, pkt)) < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
+            return ret;
+
+        if(pkt->dts == AV_NOPTS_VALUE && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
+            return AVERROR(EINVAL);
+    } else {
+        av_dlog(s, "av_interleaved_write_frame FLUSH\n");
+        flush = 1;
+    }
 
     for(;;){
         AVPacket opkt;
-        int ret= interleave_packet(s, &opkt, pkt, 0);
+        int ret= interleave_packet(s, &opkt, pkt, flush);
         if(ret<=0) //FIXME cleanup needed for ret<0 ?
             return ret;