title: !_src.empty() in function 'cv::cvtColor'
body: <pre><code>from skimage.measure import compare_ssim
  import argparse
  import imutils
  import cv2
  
  first = (&quot;original_02.png&quot;)
  second = (&quot;modified_02.png&quot;)
  
  
  imageA = cv2.imread(&quot;first&quot;)
  imageB = cv2.imread(&quot;second&quot;)
  
  
  grayA = cv2.cvtColor(imageA, cv2.COLOR_BGR2GRAY)
  grayB = cv2.cvtColor(imageB, cv2.COLOR_BGR2GRAY)
  
  
  (score, diff) = compare_ssim(grayA, grayB, full=True)
  diff = (diff * 255).astype(&quot;uint8&quot;)
  print(&quot;SSIM: {}&quot;.format(score))
  
  
  thresh = cv2.threshold(diff, 0, 255, cv2.THRESH_BINARY_INV | cv2.THRESH_OTSU)[1]
  cnts = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
  cnts = imutils.grab_contours(cnts)
  
  
  for c in cnts:
  
      (x, y, w, h) = cv2.boundingRect(c)
      cv2.rectangle(imageA, (x, y), (x + w, y + h), (0, 0, 255), 2)
      cv2.rectangle(imageB, (x, y), (x + w, y + h), (0, 0, 255), 2)
  
  
  cv2.imshow(&quot;Original&quot;, imageA)
  cv2.imshow(&quot;Modified&quot;, imageB)
  cv2.imshow(&quot;Diff&quot;, diff)
  cv2.imshow(&quot;Thresh&quot;, thresh)
  cv2.waitKey(0)
  </code></pre>
  <p>This is my code and I'm getting the error witch I have mentioned in the title.
  and I have kept code and necessary files in the same folder</p>
  <p>grayA = cv2.cvtColor(imageA, cv2.COLOR_BGR2GRAY)
  cv2.error: OpenCV(4.4.0) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-j8nxabm_\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'</p>
  <p>above is the error I'm getting</p>