Error while running code that uses Haar Classifier xml: "Incorrect type of self "

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I created my own Haar Cascade Classifier for detecting cows. The code works perfectly fine for the available OpenCv xml files but shows the following error when I run it using the xml file that I created.

Traceback (most recent call last):
File "C:........\Haar Cascade\cow_detect_image.py", line 11, in < module >

cows = cow_cascade.detectMultiScale(gray, 1.3, 5)

TypeError: Incorrect type of self (must be 'CascadeClassifier' or its derivative)

Here's the code:

import numpy as np

import cv2

cow_cascade = cv2.CascadeClassifier('cow.xml')

img = cv2.imread('Cow3.bmp')

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

cows = cow_cascade.detectMultiScale(gray, 1.3, 5)

for (x,y,w,h) in cows:

cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)

roi_gray = gray[y:y+h, x:x+w]

roi_color = img[y:y+h, x:x+w]

cv2.imshow('img',img)

cv2.waitKey(0)

cv2.destroyAllWindows()

What is wrong and how can I fix this?