An error CV2 is reported when the picture is converted to grayscale cvtColor(img, cv2.COLOR_BGR2GRAY)

Turn the picture to grayscale cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) An error is as follows ：

  img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.5.4) d:\a\opencv-python\opencv-python\opencv\modules\imgproc\src\color.simd_helpers.hpp:92: error: (-2:Unspecified error) in function '__cdecl cv::impl::`anonymous-namespace'::CvtHelper<struct cv::impl::`anonymous namespace'::Set<3,4,-1>,struct cv::impl::A0x5252fd2b::Set<1,-1,-1>,struct cv::impl::A0x5252fd2b::Set<0,2,5>,2>::CvtHelper(const class cv::_InputArray &,const class cv::_OutputArray &,int)'
> Invalid number of channels in input image:
>     'VScn::contains(scn)'
> where
>     'scn' is 1

The image should be three channels when positioning to gray conversion , Otherwise, the report will be wrong , Before converting to grayscale , Add the following code to convert the diagram into three channels

img = cv2.merge([img,img,img])

reason ：
use CV_BGR2GRAY,
Conversion formula Gray = 0.1140B + 0.5870G + 0.2989*R
Therefore, a single channel will definitely report an error