forms.BooleanField field issue

[george.li]

in django 1.4 1.5.1

from django import forms
forms.BooleanField().clean(True)

this case is success

But

forms.BooleanField().clean(False)

forms.BooleanField().clean('false')

in this two case are
* ValidationError: [u'This field is required.']

source code

 621 class BooleanField(Field):
 622     widget = CheckboxInput
 623
 624     def to_python(self, value):
 625     ¦   """Returns a Python boolean object."""
 626     ¦   # Explicitly check for the string 'False', which is what a hidden field
 627     ¦   # will submit for False. Also check for '0', since this is what
 628     ¦   # RadioSelect will provide. Because bool("True") == bool('1') == True,
 629     ¦   # we don't need to handle that explicitly.
 630     ¦   if isinstance(value, six.string_types) and value.lower() in ('false', '0'):
 631     ¦   ¦   value = False
 632     ¦   else:
 633     ¦   ¦   value = bool(value)
 634     ¦   value = super(BooleanField, self).to_python(value)
 635     ¦   if not value and self.required:
 636     ¦   ¦   raise ValidationError(self.error_messages['required'])
 637     ¦   return value

in 630, 'false' and '0' Transform to boolean False
but 635 then decide False is no input value