error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

[shreevardhan97]

Traceback (most recent call last):
File "./DetectEmotion.py", line 51, in
gray, detected_faces = detect_face(frame)
File "./DetectEmotion.py", line 29, in detect_face
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.1.0) /Users/travis/build/skvark/opencv-python/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'