error: OpenCV(4.1.2) (-215:Assertion failed) image.channels() == 1 || image.channels() == 3 || image.channels() == 4 in function 'cv::imwrite_' [duplicate]

I use openCV(4.1.2) in python 3.7 to process my photos to MNIST format in windows 7.

First I want to resize my photo to 28*28， and then convert it to grayscale.

Finally, I want to store the converted photos.

I use the following code:

def resize(img): img = cv.cvtColor(img, cv.COLOR_RGB2GRAY) h = img.shape[0] w = img.shape[1] p = max(h,w)/28 if h > w: resize_h = 28 resize_w = w/p else: resize_w = 28 resize_h = h/p img_resized = cv.resize(img, (int(resize_h), int(resize_w)), interpolation = cv.INTER_AREA) img_resized = cv.resize(img, (28, 28), interpolation = cv.INTER_AREA) return img_resized def load_data(path): idx = 0 total_imgs = len([img_name for img_name in os.listdir(path) if img_name.endswith('.PNG')]) data = np.zeros((total_imgs,28,28), dtype=np.uint8) for img_name in os.listdir(path): if not img_name.endswith('.PNG'): continue img_path = os.path.join(path, img_name) img = cv.imread(img_path) resized_img = resize(img) data[idx, :]=resized_img idx+=1 return data data = load_data('D:\\EPS_projects\\AI\\2_CV\\cifar-10\\img\\0') cv.imwrite("D:\\EPS_projects\\AI\\2_CV\\MNIST\\work200306\\0\\1_im.PNG", data); 

But when I run this piece of code, error occurs at the last line when I tried to use cv.imwrite to save the converted photos.

The error is: error: OpenCV(4.1.2) C:\projects\opencv-python\opencv\modules\imgcodecs\src\loadsave.cpp:668: error: (-215:Assertion failed) image.channels() == 1 || image.channels() == 3 || image.channels() == 4 in function 'cv::imwrite_'

How to solve my problem?