protected function create_key() {
		if ( is_array( $this->data ) ) {
			$this->data[ $this->key ] = null;
		} elseif ( is_object( $this->data ) ) {
			$this->data->{$this->key} = null;
		} else {
			$type = gettype( $this->data );
			throw new UnexpectedValueException(
				"Cannot create key \"{$this->key}\" on data type {$type}"
			);
		}
	}