title: OpenCV error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'
body: <p>I'm currently in Windows 10, using Python 3 and cv2.</p>
  
  <p>I am trying to run the following code:</p>
  
  <pre><code>import cv2
  import numpy as np
  
  img = cv2.imread('test.jpg', 1) 
  hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
  </code></pre>
  
  <p>But I get this error when I run it:</p>
  
  <pre><code>Traceback (most recent call last):
  
    File "&lt;ipython-input-177-540d1ccc2ece&gt;", line 6, in &lt;module&gt;
      hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
  
  error: OpenCV(3.4.4) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:181: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'
  </code></pre>
  
  <p>Please help. Thanks a lot in advance.</p>