os.chmod is implemented in posixmodule.c and the argument parsing code can be found at http://hg.python.org/cpython/file/3acbb23c73bc/Modules/posixmodule.c#l2605 . You will notice that the argument parsing is specified as "O&i|$O&p". That means the first argument is parsed as a Python object which is passed through a converter function and the second argument is required to be an integer. That converter function can be found at http://hg.python.org/cpython/file/3acbb23c73bc/Modules/posixmodule.c#l681. Looking at that code doesn't suggest that TypeError is raised with that message.

What were the exact arguments you passed into os.chmod() that triggered the exception?