ffmpeg bashスクリプトは、ファイル名にスペースが含まれていると字幕を使用できません

これは私の現在のスクリプトです。

#!/bin/bash


while true
do
for f in *.mkv; 

do
/home5/user/ffmpeg_folder/./ffmpeg-git-20181101-64bit-static/ffmpeg \
 -i "$f" -c:v h264 -maxrate 12M -crf 16 -c:a copy -vf subtitles="$f" -pix_fmt yuv420p \
  "${f%.mkv}".mp4; 
  done
  sleep 15m
done

このエラーは引き続き発生します。

      NUMBER_OF_BYTES-eng: 30679
      _STATISTICS_WRITING_APP: no_variable_data
      _STATISTICS_WRITING_APP-eng: no_variable_data
      _STATISTICS_WRITING_DATE_UTC: 1970-01-01 00:00:00
      _STATISTICS_WRITING_DATE_UTC-eng: 1970-01-01 00:00:00
      _STATISTICS_TAGS: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
    Stream #0:3: Attachment: ttf
    Metadata:
      filename        : OpenSans-Semibold.ttf
      mimetype        : application/x-truetype-font
File '[HorribleSubs] Seishun Buta Yarou wa Bunny Girl Senpai no Yume wo Minai - 07 [1080p].mp4' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
[Parsed_subtitles_0 @ 0x5dfe3c0] No filename provided!
[AVFilterGraph @ 0x6da2c00] Error initializing filter 'subtitles' with args ''
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #0:0
Conversion failed!

-vf subtitles="$f"それがなければ、すべてが大丈夫でしょう。この問題を解決する方法はありますか？