Show documentation fail with exception

[ntcong]

Whenever I use show documentation with SublimeRope, I see this in the console

Traceback (most recent call last):
File "./sublime_plugin.py", line 362, in run_
File "./sublime_rope.py", line 532, in run
maxfixes=3)
File "./rope/contrib/codeassist.py", line 63, in get_doc
File "./rope/contrib/codeassist.py", line 514, in get_doc
File "./rope/contrib/codeassist.py", line 552, in _get_function_docstring
File "./rope/contrib/codeassist.py", line 572, in _get_super_methods
File "./rope/contrib/codeassist.py", line 567, in _get_super_methods
AttributeError: 'PyObject' object has no attribute 'get_superclasses'

SublimeRope can go to definition just fine, but show documentation cannot work.
I'm using Linux, Python 2.7.3 and Sublime Text 2217