Numpy.copy(image array) giving zero-size array error

I have an image that is 441x269. Following up from here, I'm trying to extract sub-images from it, like this:

rect = np.copy(image[start_x:end_x, start_y:end_y]) 

But it crashes when it gets to this:

rect = np.copy (img[302:441, 31:160]) 
ig = ImageViewer(rect)
ig.show()

or if, after the np.copy command, I do this:

misc.imsave(newfile, rect)

Put together, this is the code that can be used to reproduced the problem:

from skimage.viewer import ImageViewer
import cv2
import scipy.io as sio
from skimage import data, io, color
import numpy as np
from scipy import misc

img = cv2.imread("C:\\PATH_TO_MY_IMAGES.jpg")
img = color.rgb2gray(img)

# the line below creates a problem, but if I try, for example this: 
# np.copy (img[171:441, 33:211]), then no problem

rect = np.copy (img[302:441, 31:160]) 

#try the below and it gives an error. 
ig = ImageViewer(rect)
ig.show()

newfile= "C:\\work_asaaki\\patches\\ss_test\\%d.jpg" % j
misc.imsave(newfile, rect)

Why? As far as I can see, all the given parameters fit within the image, and they form a normal rectangle.

The error I get is this:

C:\Program Files (x86)\Anaconda\lib\site-packages\matplotlib\axes.py:2760: UserWarning: Attempting to set identical bottom==top results
in singular transformations; automatically expanding.
bottom=-0.5, top=-0.5
  + 'bottom=%s, top=%s') % (bottom, top))
C:\Program Files (x86)\Anaconda\lib\site-packages\matplotlib\axes.py:2760: UserWarning: Attempting to set identical bottom==top results
in singular transformations; automatically expanding.
bottom=0, top=0
  + 'bottom=%s, top=%s') % (bottom, top))
Traceback (most recent call last):
  File "C:\Users\Administrator\.spyder2\.temp.py", line 51, in <module>
    ig = ImageViewer(rect)
  File "C:\Program Files (x86)\Anaconda\lib\site-packages\skimage\viewer\viewers\core.py", line 108, in __init__
    self._update_original_image(image)
  File "C:\Program Files (x86)\Anaconda\lib\site-packages\skimage\viewer\viewers\core.py", line 165, in _update_original_image
    self.image = image.copy()       # update displayed image
  File "C:\Program Files (x86)\Anaconda\lib\site-packages\skimage\viewer\viewers\core.py", line 235, in image
    if clim[0] < 0 and image.min() >= 0:
  File "C:\Program Files (x86)\Anaconda\lib\site-packages\numpy\core\_methods.py", line 21, in _amin
    out=out, keepdims=keepdims)
ValueError: zero-size array to reduction operation minimum which has no identity

What am I doing wrong?