creating virtualenv for django but i get import zlib module error what could be the problem

raceback (most recent call last):
  File "/usr/local/bin/virtualenv", line 9, in <module>
    load_entry_point('virtualenv==15.0.1', 'console_scripts', 'virtualenv')()
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 542, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2569, in load_entry_point
    return ep.load()
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2229, in load
    return self.resolve()
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2235, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/lib/python3/dist-packages/virtualenv.py", line 1993, in <module>
    """)
  File "/usr/lib/python3/dist-packages/virtualenv.py", line 1848, in convert
    return zlib.decompress(b).decode('utf-8')
NameError: name 'zlib' is not defined

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