def __init__(self, metadata_key, value, original_exception=None):
super(InvalidMetadata, self).__init__()<EOL>self._key = metadata_key<EOL>self._value = value<EOL>self._original_exception = original_exception<EOL>
``metadata_key`` tells which metadata has the
        invalid ``value``. If ``original_exception`` is supplied, it will be
        used to supply additional information.