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I have the following simple python code:

import numpy as np
import cv2



'''Device index is just the number to specify 
which camera. Normally one camera will be connected 
(as in my case). So I simply pass 0 (or -1)
'''

cap = cv2.VideoCapture(0)

while(True):
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

But it doesn't run, and i get the error message:

OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file C:\OpenCV-2.4.2\sources\modules\imgproc\src\color.cpp, line 3739
Traceback (most recent call last):
  File "C:\Users\dom\Dropbox\eclipse_luna\testpythen\video_capture.py", line 18, in <module>
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.error: C:\OpenCV-2.4.2\sources\modules\imgproc\src\color.cpp:3739: error: (-215) scn == 3 || scn == 4 in function cvtColor

I'm new to OpenCV so dont know how to get this code working. I've googled around this error, but am a bit lost. Could it be a problem with my color.cpp module; or the setup of my webcam?

I'm using Eclipse to edit my python scripts.