beginner errors with Raspberry PI / Opencv / Python

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• I've installed today opencv 2.4.10

• I'm using some standard code from the tutorial:

  import numpy as np import sys sys.path.append('/usr/local/lib/python2.7/site-packages')

  import cv2
  
  cap = cv2.VideoCapture(0)
  
      # 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)
  
  #cv2.imwrite("I2.jpg",img)
  
  # When everything done, release the capture
  cap.release()
  cv2.destroyAllWindows()
  

*

• I'm getting the following error messages:

  OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file /home/pi/opencv-2.4.10/modules/imgproc/src/color.cpp, line 3739 Traceback (most recent call last): File "I2_video.py", line 14, in <module> gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY) cv2.error: /home/pi/opencv-2.4.10/modules/imgproc/src/color.cpp:3739: error: (-215) scn == 3 || scn == 4 in function cvtColor