KeyError 'Home'

[Deleted user]

When using a PHP script to execute the isign command I get the following error

Traceback (most recent call last):
  File "/usr/local/bin/isign", line 4, in <module>
    from isign import isign
  File "/usr/local/lib/python2.7/dist-packages/isign/isign.py", line 37, in <module>
    if exists(join(os.environ['HOME'], 'isign-credentials')):
  File "/usr/lib/python2.7/UserDict.py", line 23, in __getitem__
    raise KeyError(key)
KeyError: 'HOME'

How can I fix this?