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your image is empty. probably the VideoCapture did not open correctly.

please check:

cam = cv2.VideoCapture(0)
if not cam.isOpened(): raise Exception("could not open camera")

and later:

    ret, im =cam.read()
    if not ret: break # no image

then, please avoid using deprecated cv2.cv things, they're entirely gone in recent opencv, use cv2.putText() and cv2.FONT_HERSHEY_SIMPLEX instead.