cv2 error OpenCV 4 5 3 tmp pip-req-build-dwj9trv5 opencv modules imgproc src color cpp 182 error -215 Assertion failed src empty in function cvtColor

Question

Traceback (most recent call last):
  File "main.py", line 86, in <module>
    image,face = face_detector(frame)
  File "main.py", line 59, in face_detector
    gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.5.3) /tmp/pip-req-build-dwj9trv5/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'