>>> import matplotlib.pyplot as plt
>>> import numpy as np
>>> plt.imshow(np.arange(12).reshape(3, 4), cmap='hots')
<matplotlib.image.AxesImage object at 0x25265d0>
>>> plt.show()
Traceback (most recent call last):
  File "matplotlib/backends/backend_gtk.py", line 423, in expose_event
    self._render_figure(self._pixmap, w, h)
  File "matplotlib/backends/backend_gtkagg.py", line 75, in _render_figure
    FigureCanvasAgg.draw(self)
  File "matplotlib/backends/backend_agg.py", line 433, in draw
    self.figure.draw(self.renderer)
  File "matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "matplotlib/figure.py", line 925, in draw
    func(*args)
  File "matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "matplotlib/axes.py", line 1998, in draw
    a.draw(renderer)
  File "matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "matplotlib/image.py", line 359, in draw
    im = self.make_image(renderer.get_image_magnification())
  File "matplotlib/image.py", line 579, in make_image
    transformed_viewLim)
  File "matplotlib/image.py", line 202, in _get_unsampled_image
    x = self.to_rgba(self._A, bytes=False)
  File "matplotlib/cm.py", line 241, in to_rgba
    x = self.cmap(x, alpha=alpha, bytes=bytes)
TypeError: 'NoneType' object is not callable