Python cString raise exception when feed Unicode

I gett following different result when using these two Class.

>>> import StringIO
>>> import cStringIO
>>> StringIO.StringIO().write(u'\u2222')
>>> cStringIO.StringIO().write(u'\u2222')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2222' in position 0: ordinal not in range(128)

The root cause is explained on document for cStringIO

Unlike the StringIO module, this module is not able to accept Unicode strings that cannot be encoded as plain ASCII strings.