OpenCV 4.4.0 error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor' , 14 other erros module cv2

Question

cap = cv2.VideoCapture(0)

while True:
    # Grab a single frame of video
    ret, frame = cap.read()
    gray = cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY)
    faces = f