In the 2.7 branch, the exception message of str.join is missing when the argument is non-sequence:

>>> ' '.join(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError

I fix this with a patch. After this:

>>> ' '.join(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only join an iterable

I also add test to this case. Can the test also be added to 3.x branches?