Actually, it looks like ANY warning will raise this error in IDLE.  For
example:

>>> import warnings
>>> warnings.warn('blah blah')

Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    warnings.warn('blah blah')
  File "C:\Python26\Lib\warnings.py", line 29, in _show_warning
    file.write(formatwarning(message, category, filename, lineno, line))
TypeError: idle_formatwarning_subproc() takes exactly 4 arguments (5 given)