TypeError for simple object schema with additionalProperties=false

[bdziendzik]

Running validation for a schema like:

{
"main": {
              "enum": [
                "duck",
                "chicken",
                "beef"
              ],
              "type": "string",
              "additionalProperties": false,
              "description": "Status of current or last loan."
            }
}

will raise

...
File "/Users/bartosz/.virtualenvs/django/lib/python2.7/site-packages/flex/validation/common.py", line 384, in validate_object
    schema_validators = construct_schema_validators(schema, context)
  File "/Users/bartosz/.virtualenvs/django/lib/python2.7/site-packages/flex/validation/schema.py", line 230, in construct_schema_validators
    generate_additional_properties_validator(context=context, **schema),
TypeError: generate_additional_properties_validator() takes exactly 2 arguments (1 given)

The generate_additional_properties_validator is called if schema.get('additionalProperties') is False is met and it expects properties of the schema to be passed but there is no properties in string field.