diff --git a/libavformat/amr.c b/libavformat/amr.c
index d16b4c03cd..f78aacc216 100644
--- a/libavformat/amr.c
+++ b/libavformat/amr.c
@@ -100,12 +100,14 @@ static int amr_read_header(AVFormatContext *s,
         st->codec->codec_tag = MKTAG('s', 'a', 'w', 'b');
         st->codec->codec_id = CODEC_ID_AMR_WB;
         st->codec->sample_rate = 16000;
+        st->codec->frame_size = 320;
     }
     else
     {
         st->codec->codec_tag = MKTAG('s', 'a', 'm', 'r');
         st->codec->codec_id = CODEC_ID_AMR_NB;
         st->codec->sample_rate = 8000;
+        st->codec->frame_size = 160;
     }
     st->codec->channels = 1;
     st->codec->codec_type = AVMEDIA_TYPE_AUDIO;