Virtualenv error

$  virtualenv venv
Traceback (most recent call last):
  File "/home/rohit/bin/virtualenv", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 2880, in <module>
    parse_requirements(__requires__), Environment()
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 596, in resolve
    raise DistributionNotFound(req)
pkg_resources.DistributionNotFound: virtualenv==1.9.1