$dbalter->add_field Question

[TheMayhem]

I'm working on finishing up a product I've created and am having a tiny bit of issues with the $dbalter->add_field function. I need to add a field to the usergroup database that can contain decimals. I've tried Double & Decimal (12,2) but haven't had much success. All other fields I've added have worked correctly except when it comes to decimal manipulation. Here is a copy of my code:

PHP Code:

// Make vB table alterations
require_once(DIR . '/includes/class_dbalter.php');
$dbalter = new vB_Database_Alter_MySQL($db);

$dbalter->add_field(array(
    'name' => 'market_discount',
    'type' => 'DOUBLE',
    'length' => 8,
    'attributes' => 'UNSIGNED',
    'null' => false,
    'default' => '0'
));
$db->show_errors(); 


What can I do differently to get the double or even a decimal data type in there to work correctly?