Firebird cannot handle DECIMAL(255, 0)

SQL ERROR [ firebird ]

Dynamic SQL Error SQL error code = -842 Precision must be from 1 to 18 [-842]

SQL

UPDATE phpbb_config SET config_value = CAST(CAST(config_value as DECIMAL(255, 0)) + 1 as VARCHAR(255)) WHERE config_name = 'num_posts'

/**

* Set dynamic config value with arithmetic operation.

*/

function set_config_count($config_name, $increment, $is_dynamic = false)

{

        global $db, $cache;

        switch ($db->sql_layer)

        {

                case 'firebird':

                        $sql_update = 'CAST(CAST(config_value as DECIMAL(18, 0)) + ' . (int) $increment . ' as VARCHAR(255))';