Submission gives error with possesive case

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I am trying to construct this term:

http://laktoz.yogurtcultures.org/?subject=ahmet&case=possesive&predicate=ev&whom=first_person_singular

After submission page gives an error with the stacktrace:

builtins.TypeError
TypeError: possesive() missing 1 required positional argument: 'person'

Traceback (most recent call last)
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
return self.wsgi_app(environ, start_response)
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
response = self.handle_exception(e)
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
reraise(exc_type, exc_value, tb)
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
raise value
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
response = self.full_dispatch_request()
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
raise value
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
rv = self.dispatch_request()
File "/home/arguman/env/laktoz/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/home/arguman/laktoz/app.py", line 423, in index
result = render_result(request.args)
File "/home/arguman/laktoz/app.py", line 352, in render_result
parts.append((subject_case, case_processor(subject)[len(subject):], colorize(case_processor)))
TypeError: possesive() missing 1 required positional argument: 'person'
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Either formation does not make sense, or there is a bug. In any case, it should not give a code error.