diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index 632ac25cb2..1089cc48c4 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -1118,7 +1118,7 @@ static void do_subtitle_out(OutputFile *of,
             exit_program(1);
         }
 
-        pkt->size = subtitle_out_size;
+        av_shrink_packet(pkt, subtitle_out_size);
         pkt->pts  = av_rescale_q(sub->pts, AV_TIME_BASE_Q, ost->mux_timebase);
         pkt->duration = av_rescale_q(sub->end_display_time, (AVRational){ 1, 1000 }, ost->mux_timebase);
         if (enc->codec_id == AV_CODEC_ID_DVB_SUBTITLE) {