diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 4a4c240a7f..62cb68086f 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -312,6 +312,7 @@ typedef struct AVStream {
      * decoding: pts of the first frame of the stream, in stream time base.
      * only set this if you are absolutely 100% sure that the value you set
      * it to really is the pts of the first frame
+     * This may be undefined (AV_NOPTS_VALUE).
      * @note the ASF header does NOT contain a correct start_time the ASF
      * demuxer must NOT set this
      */