Django模型选择不会因无效选择而引发错误

我在Django中有一个带有选择字段的对象

class CustomFieldType(models.Model):
    STRING = 'STRING'
    DATE = 'DATE'
    BOOLEAN = 'BOOLEAN'
    NUMERIC = 'NUMERIC'
    EMAIL = 'EMAIL'
    TYPE_CHOICES = (
        (STRING, _('String')),
        (DATE, _('Date')),
        (BOOLEAN, _('Boolean')),
        (NUMERIC, _('Numeric')),
        (EMAIL, _('Email'))
    )
    name = models.CharField(max_length=256)
    field_type = models.CharField(choices=TYPE_CHOICES, default=STRING, max_length=10)
    company = models.ForeignKey('Company')

    class Meta:
        unique_together = ('name', 'company')

    def __unicode__(self):
        return self.name

$> CustomFieldType.objects.create(name='custom_name',field_type='noError',company=mycompany)

$> CustomFieldType.objects.get(name='custom_name').field_type
u'noError'