(-215:Assertion failed) src.type() == CV_8UC1 in function 'cv::adaptiveThreshold' code example

Example 1: (-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)

Example 2: open cverror: (-215:Assertion failed) src.type() == CV_8UC1 in function 'cv::adaptiveThreshold'

from keras.preprocessing import image
import cv2
import matplotlib.pyplot as plt

img = image.load_img('15f8U.png', grayscale=True, target_size=(224, 224))
img = image.img_to_array(img, dtype='uint8')

print(img.shape)
## output : (224,224,3)
#plt.imshow(img_grey)

th3 = cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C,cv2.THRESH_BINARY,11,2)
plt.figure(figsize=(20,10))
plt.imshow(th3, cmap="gray")
plt.show()