ValueError invalid literal for int() with base 10: ''

Raised by:

django/db/models/fields/__init__.py (line 1853)

Traceback (most recent call last):
  File "python3.5/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "python3.5/site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "python3.5/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "private.py", line 1, in private_function
    private_function()
  File "python3.5/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "python3.5/site-packages/django/db/models/query.py", line 784, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "python3.5/site-packages/django/db/models/query.py", line 802, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "python3.5/site-packages/django/db/models/sql/query.py", line 1250, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "python3.5/site-packages/django/db/models/sql/query.py", line 1270, in _add_q
    current_negated, allow_joins, split_subq)
  File "python3.5/site-packages/django/db/models/sql/query.py", line 1276, in _add_q
    allow_joins=allow_joins, split_subq=split_subq,
  File "python3.5/site-packages/django/db/models/sql/query.py", line 1210, in build_filter
    condition = self.build_lookup(lookups, col, value)
  File "python3.5/site-packages/django/db/models/sql/query.py", line 1104, in build_lookup
    return final_lookup(lhs, rhs)
  File "python3.5/site-packages/django/db/models/lookups.py", line 24, in __init__
    self.rhs = self.get_prep_lookup()
  File "python3.5/site-packages/django/db/models/lookups.py", line 74, in get_prep_lookup
    return self.lhs.output_field.get_prep_value(self.rhs)
  File "python3.5/site-packages/django/db/models/fields/__init__.py", line 1853, in get_prep_value
    return int(value)
ValueError: invalid literal for int() with base 10: ''