Read a save for numpy array of grayscale, want to put the single channel image is converted into a three-channel, use cvtColor has been an error, the code is shown in figure

An error as shown in figure

Have tried to use directly cv2. Imread (path, 0) read a greyscale image, shape and I read numpy array, using cvtcolor can normal use, don't know where is the reason, strives for the leaders of all stripes help me have a look