[Python] Writing a video frame by frame using ffmpeg

I am trying to write a video frame by frame using ffmpeg as explained here: http://zulko.github.io/blog/2013/09/27/read-and-write-video-frames-in-python-using-ffmpeg/

However, I am always getting an OSError: [Errno 22] Invalid argument. I am on Windows 7 and using Python 3.4. Here is the code:

import subprocess as sp
import numpy as np
import time

ffmpeg_bin = r'C:\path\to\ffmpeg\ffmpeg.exe'

command = [ffmpeg_bin,
'-y', # (optional) overwrite output file if it exists
'-f', 'rawvideo',
'-vcodec','rawvideo',
'-s', '420x360', # size of one frame
'-pix_fmt', 'rgb24',
'-r', '24', # frames per second
'-i', '-', # The imput comes from a pipe
'-an', # Tells FFMPEG not to expect any audio
'-vcodec', 'mpeg',
'my_output_videofile.mp4' ]

proc = sp.Popen(command, stdin=sp.PIPE, stderr=sp.PIPE)

a = np.zeros((360, 420, 3), dtype=np.uint8)

for ii in range(5*24):
print(ii)
proc.stdin.write(a.tostring())
time.sleep(1/24)

proc.stdin.close()
proc.stderr.close()
proc.wait()


Any help is greatly appreciated.

EDIT: As requested here the detailed output of the program

0
1
Traceback (most recent call last):
File ".\write_dummy_video.py", line 25, in <module>
proc.stdin.write(a.tostring())
OSError: [Errno 22] Invalid argument


Interestingly, if I comment line number 26 (i.e. time.sleep(1/24)) the error message changes slightly but the loop still only runs twice. Here is the error output:

0
1
Traceback (most recent call last):
File ".\write_dummy_video.py", line 25, in <module>
proc.stdin.write(a.tostring())
BrokenPipeError: [Errno 32] Broken pipe

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