title: How can solve this error regarding -215 assertion error?
body: <p>Assertion failed -215 opencv</p>
  <p>!_src.empty() in function 'cv::cvtColor</p>
  <pre><code>error                                     Traceback (most recent call last)
  &lt;ipython-input-4-22f9613d8d9c&gt; in &lt;module&gt;
       13 while 1:
       14     ret, img = cap.read()
  ---&gt; 15     gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
       16     faces = face_cascade.detectMultiScale(gray, 1.3, 5)
       17 
  
  error: OpenCV(4.4.0) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-k8sx3e60\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'
  </code></pre>