Error while running video

edit

When I try to run a video I get the following error. The video runs then the error occurs. I am unable to find any information about this error.
\modules\imgproc\src\color.cpp:3961: error: (-215) (scn == 3 || scn == 4) && (depth == CV_8U || depth == CV_32F) in function cv::cvtColor

The code I am running is

    greenLower = np.array([26, 86, 6],dtype=np.uint8)
    greenUpper = np.array([90, 255, 255],dtype=np.uint8)

    cap =  cv2.VideoCapture('E:/PYTHON SCRIPTS/Learning python/metronome1.mp4')
    print cap.grab()
    while(cap.isOpened()):
      ret, frame = cap.read()

      gray = cv2.cvtColor(frame, cv2.COLOR_RGB2HSV)
      mask = cv2.inRange(gray, greenLower, greenUpper)
      mask = cv2.erode(mask, None, iterations=2)
      mask = cv2.dilate(mask, None, iterations=2)
      cv2.imshow('duck',mask)
      if cv2.waitKey(1) & 0xFF == ord('q'):
          break

Thanks