diff --git a/libavcodec/libtwolame.c b/libavcodec/libtwolame.c
index def5feeb9d..7f7b2f7590 100644
--- a/libavcodec/libtwolame.c
+++ b/libavcodec/libtwolame.c
@@ -148,8 +148,8 @@ static int twolame_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     if (ret < 0)  // twolame error
         return AVERROR_UNKNOWN;
 
-    avpkt->duration = ff_samples_to_time_base(avctx, frame->nb_samples);
     if (frame) {
+        avpkt->duration = ff_samples_to_time_base(avctx, frame->nb_samples);
         if (frame->pts != AV_NOPTS_VALUE)
             avpkt->pts = frame->pts - ff_samples_to_time_base(avctx, avctx->delay);
     } else {