python调用cv2.drawContours出现 (-215:Assertion failed) cn <= 4 in function 'cv::scalarToRawData'

python使用cv2.drawContours来绘制轮廓

cv2.drawContours(image=img, contours=[poly], contourIdx=-1, color=color1, thickness=2*tl)

img为单通道，在图像上绘制的轮廓都是黑白两色

图像融合

image = np.asarray(img)
imagemerge = cv2.merge([image, image, image])
再次运行程序出现下面错误：

cv2.error: OpenCV(4.1.2) C:\projects\opencv-python\opencv\modules\core\src\array.cpp:3229: error: (-215:Assertion failed) cn <= 4 in function 'cv::scalarToRawData'