UserWarning while running sample code for class-based Validator

I have copied an example code for class-based Validator from here : http://cerberus-sanhe.readthedocs.io/customize.html#class-validator

from cerberus import Validator

class MyValidator(Validator):
    def _validate_isodd(self, isodd, field, value):
        if isodd and not bool(value & 1):
            self._error(field, "Must be an odd number")

Unfortunately the code emits user warning: (python3.5, cerberus 1.1)

/usr/local/lib/python3.5/dist-packages/cerberus/validator.py:1338: UserWarning: No validation schema is defined for the arguments of rule 'isodd'
  "'%s'" % method_name.split('_', 2)[-1])

[olegsv]

@funkyfuture turns out the docstring is important. The following code works:

class MyValidator(Validator):
    def _validate_isodd(self, isodd, field, value):
        """ Test the oddity of a value.
        The rule's arguments are validated against this schema:
        {'type': 'boolean'}
        """
        if isodd and not bool(value & 1):
            self._error(field, "Must be an odd number")

[D-stefaang]

Note that you need to include this text literally

        The rule's arguments are validated against this schema: