A failing doctest

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File "vtodo/listing/filler.py", line 75, in filler.get_css_class_dict
Failed example:
    d['a']
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python3.6/doctest.py", line 1330, in __run
        compileflags, 1), test.globs)
      File "<doctest filler.get_css_class_dict[2]>", line 1, in <module>
        d['a']
    TypeError: 'NoneType' object is not subscriptable
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File "vtodo/listing/filler.py", line 77, in filler.get_css_class_dict
Failed example:
    d['b']
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python3.6/doctest.py", line 1330, in __run
        compileflags, 1), test.globs)
      File "<doctest filler.get_css_class_dict[3]>", line 1, in <module>
        d['b']
    TypeError: 'NoneType' object is not subscriptable
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1 items had failures:
   2 of   4 in filler.get_css_class_dict
***Test Failed*** 2 failures.