OpenCV(4.1.2) C:\projects\opencv-python\opencv\modules\imgproc\src\thresh.cpp:1645: error: (-215:Assertion failed) src.type() == CV_8UC1 in function 'cv::adaptiveThreshold' code example

Example: (-215:Assertion failed) img.type() == CV_8UC1 in func

img_blur = cv2.medianBlur(self.cropped_img,5).astype('uint8')
img_thresh_Gaussian = cv2.adaptiveThreshold(img_blur, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY, 11, 2)