The test case comparison only works when the outcome of your function is a string. In your case it is a 'NonType' object. Since "NoneType" object doesn't have certain attributes (e.g.: it doesn't have 'replace' attribute), the evaluator fails to run the test case comparison.

In a nutshell, please make sure your function returns a string

I have the exact same issue

Traceback (most recent call last):
File "main.py", line 3, in
compare_bc(generate_bc("mysite.com/pictures/holidays.html", " : "), 'HOME : PICTURES : HOLIDAYS')
File "/home/codewarrior/setup.py", line 4, in
compare_bc = lambda result,expected, msg="": Test.assert_equals(result,expected) if (result==expected) else Test.expect(result==expected,"".join(["Expected: ",expected.replace("<","<").replace(">",">")," - instead got: ", result.replace("<","<").replace(">",">")]));
AttributeError: 'NoneType' object has no attribute 'replace'

what is the problem?