protected function _get_string_value( $key ) {
		if ( ! isset( $this->types[ $key ] ) || 'string' !== $this->types[ $key ] ) {
			return null;
		}

		$default = array_key_exists( $key, $this->defaults ) ? $this->defaults[ $key ] : '';

		return trim( (string) $this->store->get( $key, $default ) );
	}