Trying to convert 'BGR to Gray'

edit

This is the input code:

import cv2
img = cv2.imread('test.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

And the error :

error: OpenCV(3.4.2) c:\miniconda3\conda-bld\opencv-suite_1534379934306\work\modules\imgproc\src\color.hpp:253: error: (-215:Assertion failed) VScn::contains(scn) && VDcn::contains(dcn) && VDepth::contains(depth) in function 'cv::CvtHelper<struct cv::set&lt;3,4,-1="">,struct cv::Set<1,-1,-1>,struct cv::Set<0,2,5>,2>::CvtHelper'C:\fakepath\test.jpg