title: skimage's rgb2gray in python: AttributeError: Nonetype object has no attribute ndim
body: <p>I was using this code (with skimage version 0.10.0) as far as I can remember without issues:</p>
  
  <pre><code>from scipy import misc
  import scipy.io as sio
  from skimage.color import rgb2gray
  
  img = cv2.imread(myfile)
  img = rgb2gray(img)
  </code></pre>
  
  <p>But now I'm getting this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\work_asaaki\code\generateProposals.py", line 48, in &lt;module&gt;
      img = rgb2gray(img)
    File "C:\Anaconda\lib\site-packages\skimage\color\colorconv.py", line 635, in rgb2gray
      if rgb.ndim == 2:
  AttributeError: 'NoneType' object has no attribute 'ndim'
  </code></pre>
  
  <p>What could the problem possibly be? How can I fix it to be able to convert the image to grayscale?</p>