title: TypeError: execv() arg 2 must contain only strings using docopt
body: <p>I am running into following error while passing arguments. Can anyone help me identify this problem? I'm passing instructions to my function in terminal like this:</p>
  
  <pre><code>python makeQuicktime.py -i /Volumes/P003A/TM_Cloud/Nagrania/Karta_04/XDROOT/Clip/D004C010_141026MM.MXF -f 25 -c prores
  </code></pre>
  
  <p>This is the <strong>main</strong> function - as you see I'v tried to pass argument --codec as string:</p>
  
  <pre><code>if __name__ == "__main__":
      args = docopt(__doc__, version='makeQuicktime 0.0.1')
      print args
      cmd_args = ""
      codec=str(args['--codec'])
  
      makeQuicktime( args['--input'], fps=args['--fps'], codec=str(args['--codec']) )
  
      os._exit(0)
  </code></pre>
  
  <p>The part of other function that this code is running (makeProRes, line 110 is the output variable):</p>
  
  <pre><code>subprocess.call([FFMPEG_PATH, 'i', input,
      '-start_number', start_frame, '-r', fps,
      '-c:v', 'prores',
      '-profile:v', '2',
      '-c:a', 'copy',
      '-threads', cpus,
      output
    ])
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "makeQuicktime.py", line 123, in &lt;module&gt;
      makeQuicktime( args['--input'], fps=args['--fps'], codec=str(args['--codec']) )
    File "makeQuicktime.py", line 53, in makeQuicktime
      makeProRes(1, input, fps, output)
    File "makeQuicktime.py", line 110, in makeProRes
      output
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 524, in call
      return Popen(*popenargs, **kwargs).wait()
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 711, in __init__
      errread, errwrite)
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1308, in _execute_child
      raise child_exception
  TypeError: execv() arg 2 must contain only strings
  </code></pre>