diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
index 63ca542337..3da5bc4f7c 100644
--- a/fftools/ffmpeg.h
+++ b/fftools/ffmpeg.h
@@ -816,7 +816,7 @@ int enc_alloc(Encoder **penc, const AVCodec *codec);
 void enc_free(Encoder **penc);
 
 int enc_open(OutputStream *ost, AVFrame *frame);
-void enc_subtitle(OutputFile *of, OutputStream *ost, AVSubtitle *sub);
+void enc_subtitle(OutputFile *of, OutputStream *ost, const AVSubtitle *sub);
 void enc_frame(OutputStream *ost, AVFrame *frame);
 void enc_flush(void);
 
diff --git a/fftools/ffmpeg_enc.c b/fftools/ffmpeg_enc.c
index f42f4055d6..4029501313 100644
--- a/fftools/ffmpeg_enc.c
+++ b/fftools/ffmpeg_enc.c
@@ -462,7 +462,7 @@ static int check_recording_time(OutputStream *ost, int64_t ts, AVRational tb)
     return 1;
 }
 
-void enc_subtitle(OutputFile *of, OutputStream *ost, AVSubtitle *sub)
+void enc_subtitle(OutputFile *of, OutputStream *ost, const AVSubtitle *sub)
 {
     Encoder *e = ost->enc;
     int subtitle_out_max_size = 1024 * 1024;