The problem is easily reproducible. Although it shouldn't give that error (and that can be fixed), it seems to me that IDLE should not be trying to give a calltip in that context. What it is trying to do is display the __doc__ attribute of the string but the __doc__ is really for the str() constructor:
>>> 'a'.__doc__
"str(string[, encoding[, errors]]) -> str\n\nCreate a new string object from the given encoded string.\nencoding defaults to the current default string encoding.\nerrors can be 'strict', 'replace' or 'ignore' and defaults to 'strict'."