Accepts: str, unicode
        Returns: unicode

def validate(self, value):
        """
        Accepts: str, unicode
        Returns: unicode
        """
        val = value
        if isinstance(val, str):
            #FIXME: unsafe decoding
            val = unicode(value)
        val = super(String, self).validate(val)
        if not isinstance(val, unicode):
            raise ValueError("Not a string: %r" % (value, ))
        return val