diff --git a/yt_dlp/postprocessor/ffmpeg.py b/yt_dlp/postprocessor/ffmpeg.py
index 1d21ffaf2..be6cc9f09 100644
--- a/yt_dlp/postprocessor/ffmpeg.py
+++ b/yt_dlp/postprocessor/ffmpeg.py
@@ -802,7 +802,7 @@ class FFmpegSubtitlesConvertorPP(FFmpegPostProcessor):
                 }
 
             info['__files_to_move'][new_file] = replace_extension(
-                info['__files_to_move'][old_file], new_ext)
+                info['__files_to_move'][sub['filepath']], new_ext)
 
         return sub_filenames, info