diff --git a/youtube_dl/downloader/external.py b/youtube_dl/downloader/external.py
index 127a92d200..e13cf547d1 100644
--- a/youtube_dl/downloader/external.py
+++ b/youtube_dl/downloader/external.py
@@ -6,7 +6,10 @@ import sys
 import re
 
 from .common import FileDownloader
-from ..compat import compat_setenv
+from ..compat import (
+    compat_setenv,
+    compat_str,
+)
 from ..postprocessor.ffmpeg import FFmpegPostProcessor, EXT_TO_OUT_FORMATS
 from ..utils import (
     cli_option,
@@ -272,7 +275,7 @@ class FFmpegFD(ExternalFD):
         args += ['-i', url, '-c', 'copy']
 
         if self.params.get('test', False):
-            args += ['-fs', compat_str(self._TEST_FILE_SIZE)] # -fs limit_size (output), expressed in bytes
+            args += ['-fs', compat_str(self._TEST_FILE_SIZE)]
 
         if protocol in ('m3u8', 'm3u8_native'):
             if self.params.get('hls_use_mpegts', False) or tmpfilename == '-':