[Python] "Error exception must derive from BaseException" when inheriting from BaseException...

What's wrong with the following code (under Python 2.7.1):

class TestFailed(BaseException):
def __new__(self, m):
self.message = m

def __str__(self):
return self.message

try:
raise TestFailed('Oops')
except TestFailed as x:
print x


When I run it, I get:

Traceback (most recent call last):
File "x.py", line 9, in <module>
raise TestFailed('Oops')
TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType


But it looks to me that TestFailed does derive from BaseException.

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