Problem in example 3b

[HOlmi-Graham]

Hello, I am getting the following error when running example 3b, and I am not sure how to fix it:
Traceback (most recent call last):
File "", line 12, in
File "C:\Users\bluecrab\Desktop\tracktor-master\tracktor\tracktor.py", line 71
, in detect_and_draw_contours
img = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
UnboundLocalError: local variable 'img' referenced before assignment