BooleanField validation issue?

The following simple code shows the boolean field is not been validated.
Is there anything wrong?

import json
from mongoengine import Document
from mongoengine import BooleanField


class Validator(Document):
    a_bool_field = BooleanField()


body_json = {
  "a_bool_field": "fffffff"
}
validator = Validator.from_json(json.dumps(body_json))
try:
    validator.validate()
except Exception as e:
    print(e.to_dict())

There's no any errors printed.

Update

I found BooleanField to_python will cause Class.from_json() convert everything not None to True,
I think the behavior is odd.

class BooleanField(BaseField):
"""Boolean field type.
.. versionadded:: 0.1.2
"""

def to_python(self, value):
    try:
        value = bool(value)
    except ValueError:
        pass
    return value

def validate(self, value):
    if not isinstance(value, bool):
        self.error('BooleanField only accepts boolean values')

Start with BooleanField.to_python and validate, then trace how Document.from_json passes the parsed value into validation. Run the supplied reproduction and verify that an invalid string is not silently accepted as True; preserve or add coverage for the expected validation behavior.