diff --git a/youtube_dl/postprocessor/ffmpeg.py b/youtube_dl/postprocessor/ffmpeg.py
index 602e370f41..45328ed43e 100644
--- a/youtube_dl/postprocessor/ffmpeg.py
+++ b/youtube_dl/postprocessor/ffmpeg.py
@@ -9,6 +9,7 @@ from .common import AudioConversionError, PostProcessor
 from ..utils import (
     check_executable,
     compat_subprocess_get_DEVNULL,
+    encodeArgument,
     encodeFilename,
     PostProcessingError,
     prepend_extension,
@@ -48,7 +49,7 @@ class FFmpegPostProcessor(PostProcessor):
         for path in input_paths:
             files_cmd.extend(['-i', encodeFilename(path, True)])
         cmd = ([self._get_executable(), '-y'] + files_cmd
-               + opts +
+               + [encodeArgument(o) for o in opts] +
                [encodeFilename(self._ffmpeg_filename_argument(out_path), True)])
 
         if self._downloader.params.get('verbose', False):
diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index aa9347f431..b97e62ae93 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -540,6 +540,16 @@ def encodeFilename(s, for_subprocess=False):
         encoding = 'utf-8'
     return s.encode(encoding, 'ignore')
 
+
+def encodeArgument(s):
+    if not isinstance(s, compat_str):
+        # Legacy code that uses byte strings
+        # Uncomment the following line after fixing all post processors
+        #assert False, 'Internal error: %r should be of type %r, is %r' % (s, compat_str, type(s))
+        s = s.decode('ascii')
+    return encodeFilename(s, True)
+
+
 def decodeOption(optval):
     if optval is None:
         return optval