Issues with using VideoWriter on opencv python

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I have been trying to get the openCV python VideoWriter object to work with no success. What I am trying to do is read a video, grab the frame, do some processing and write it back to a video file. The error that I get is:

Traceback (most recent call last):
File "detect.py", line 35, in <module>
    out_video.write(processed)
cv2.error: /tmp/opencv-z9Pa/opencv-2.4.9/modules/imgproc/src/color.cpp:4419: 
error: (-215) src.depth() == dst.depth() in function cvCvtColor

The code I wrote is as follows:

video = VideoCapture(args["video"])
num_frames = video.get(CV_CAP_PROP_FRAME_COUNT)
width = video.get(CV_CAP_PROP_FRAME_WIDTH)
height = video.get(CV_CAP_PROP_FRAME_HEIGHT)

fps = video.get(CV_CAP_PROP_FPS)

out_video = VideoWriter()
fourcc = CV_FOURCC('m', 'p', '4', 'v')
out_video.open(args["out"], fourcc, int(fps), (int(width), int(height)),True)

while (video.isOpened()):
    ret, frame = video.read()
    # This simply takes the frame and does some image processing on it
    segments = compute_superpixels(frame, num_pixels=100)
    processed = mark_boundaries(frame, segments)
    out_video.write(processed)

Does anyone have any idea what I might be doing wrong here? I am on Mac OSX.

I tried something which might shed some light (or not). So, if I want to write the original frame i.e. replace

out_video.write(processed)

with

out_video.write(frame)

I get my original video back. However, the frame and processed object have the same size and type! So, now I am completely baffled as to what is going on. The output of processed and frame shape and types are:

frame: (576, 720, 3)
processed: (576, 720, 3)
frame: <type 'numpy.ndarray'>
processed: <type 'numpy.ndarray'>