Function definition in mariadb 10.11.13 syntax error - typical message for a missing semicolon, but the semicolon is there [closed]

DELIMITER // 

CREATE FUNCTION total_cost_for_customer (
    custNumber INT
)
RETURNS INT
DETERMINISTIC
READS SQL DATA
BEGIN
    DECLARE total INT;

    SELECT SUM(unitPrice)
    INTO total
    FROM OrderLine
    WHERE orderNo IN (
        SELECT orderNo
        FROM FoodOrder
        WHERE custNo = custNumber
    );

    RETURN total;
END //

DELIMITER ;

SQL Error [1064] [42000]: (conn=18) You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '' at line 8

Line 8 would be the DECLARE statement.