diff --git a/libavformat/assenc.c b/libavformat/assenc.c
index bde1096269..079991d14e 100644
--- a/libavformat/assenc.c
+++ b/libavformat/assenc.c
@@ -84,7 +84,7 @@ static void purge_dialogues(AVFormatContext *s, int force)
         avio_printf(s->pb, "Dialogue: %s\r\n", dialogue->line);
         if (dialogue == ass->last_added_dialogue)
             ass->last_added_dialogue = next;
-        av_free(dialogue->line);
+        av_freep(&dialogue->line);
         av_free(dialogue);
         if (next)
             next->prev = NULL;