Use computeNMChannels with the ERFILTER_NM_IHSGRAD option

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Hello

I am trying to use the function computeNMChannels with the ERFILTER_NM_IHSGRAD option instead of the ERFILTER_NM_RGBLGrad default one in a python script. The following line throws an error:

channels = cv2.text.computeNMChannels(img, cv2.text.ERFILTER_NM_IHSGRAD)  
--> SystemError: error return without exception set

Can any one help me? How can Ii use this option?