protected function assert_comparator( $operator ) {
		if ( is_bool( $operator ) ) {
			return;
		}

		if ( in_array( $operator, array(
			'=',
			'!=',
			'>',
			'<',
			'>=',
			'<=',
			'<=>',
			'<>',
			'LIKE',
			'BETWEEN',
			'COALESCE',
			'GREATEST',
			'IN',
			'INTERVAL',
			'IS',
			'IS NOT',
			'IS NOT NULL',
			'IS NULL',
			'ISNULL',
			'LEAST',
			'NOT BETWEEN',
			'NOT IN',
			'NOT LIKE',
		), true ) ) {
			return;
		}

		throw new \InvalidArgumentException( sprintf( 'Invalid SQL operator % s . ', $operator ) );
	}