diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index d906662b95..f275d44e55 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -603,6 +603,7 @@ static int mkv_write_tracks(AVFormatContext *s)
                     int d_width = codec->width*av_q2d(st->sample_aspect_ratio);
                     put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , d_width);
                     put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, codec->height);
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYUNIT, 3);
                 }
                 end_ebml_master(pb, subinfo);
                 break;