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How to find contour of black regions in white background ?

I have image like this, I want to find contours of black regions. How can I do it ?
I use following code

                 im = cv2.imread(img)
                    plt.imshow(im)
                    print(type(im))
                    gray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
                    blurred = cv2.GaussianBlur(gray, (5, 5), 0)
                    thresh = cv2.threshold(im, 60, 255, cv2.THRESH_BINARY_INV)
                    cnts = cv2.findContours(thresh, cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

How ever it gives an error like "TypeError: Expected Ptr<cv::umat> for argument 'image' ". Can you help me solve it ? Thnaks image description