title: imgHsv = cv2.cvtColor(img,cv2.COLOR_BGR2HSV) cv2.error: OpenCV(4.0.0)
body: <p>I have been trying to run a color detection on my webcam by using OpenCV, and it keeps getting the error as shown in the title, and below:</p>
  
  <pre><code> imgHsv = cv2.cvtColor(img,cv2.COLOR_BGR2HSV)
  cv2.error: OpenCV(4.0.0) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:181: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'
  </code></pre>
  
  <p>The code that I wanted to run is:</p>
  
  <pre><code>import cv2
  import numpy as np
  
  frameWidth = 640
  frameHeight = 480
  cap = cv2.VideoCapture(1)
  cap.set(3, frameWidth)
  cap.set(4, frameHeight)
  
  def empty(a):
      pass
  
  cv2.namedWindow("HSV")
  cv2.resizeWindow("HSV",640,240)
  cv2.createTrackbar("HUE Min","HSV",0,179,empty)
  cv2.createTrackbar("HUE Max","HSV",179,179,empty)
  cv2.createTrackbar("SAT Min","HSV",0,255,empty)
  cv2.createTrackbar("SAT Max","HSV",255,255,empty)
  cv2.createTrackbar("VALUE Min","HSV",0,255,empty)
  cv2.createTrackbar("VALUE Max","HSV",255,255,empty)
  
  while True:
  
      _, img = cap.read(0)
      imgHsv = cv2.cvtColor(img,cv2.COLOR_BGR2HSV)
  
      h_min = cv2.getTrackbarPos("HUE Min","HSV")
      h_max = cv2.getTrackbarPos("HUE Max", "HSV")
      s_min = cv2.getTrackbarPos("SAT Min", "HSV")
      s_max = cv2.getTrackbarPos("SAT Max", "HSV")
      v_min = cv2.getTrackbarPos("VALUE Min", "HSV")
      v_max = cv2.getTrackbarPos("VALUE Max", "HSV")
      print(h_min)
  
      lower = np.array([h_min,s_min,v_min])
      upper = np.array([h_max,s_max,v_max])
      mask = cv2.inRange(imgHsv,lower,upper)
      result = cv2.bitwise_and(img,img, mask = mask)
  
      mask = cv2.cvtColor(mask, cv2.COLOR_GRAY2BGR)
      hStack = np.hstack([img,mask,result])
      cv2.imshow('Original', img)
      cv2.imshow('HSV Color Space', imgHsv)
      cv2.imshow('Mask', mask)
      cv2.imshow('Result', result)
      cv2.imshow('Horizontal Stacking', hStack)
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          break
  
  cap.release()
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>I am not sure whether this is the issue of OpenCV version, or anything related to the code or smth. Any ideas?</p>