Problem with 'Allowed Values' for a decimal field.

Okay, so I have a CCK decimal field that needs to only be allowed to be one of the following values: 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0.

However, entering those values into the 'Allowed Values' box results in the following error when I try to change set the field's value to, say, 4:

* warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /home/squashju/public_html/dev/sites/default/modules/cck/modules/number/number.module on line 236.

* Skill Level: illegal value.

The offending code is the following line:

if (count($allowed_values) && !array_key_exists($item['value'], $allowed_values)) {
              form_set_error($error_element, t('%name: illegal value.', array('%name' => t($field['widget']['label']))));
            }

A printout of $allowed_values and a printout of $items produce the following arrays:

$allowed_values: Array ( [2.0] => 2.0 [2.5] => 2.5 [3.0] => 3.0 [3.5] => 3.5 [4.0] => 4.0 [4.5] => 4.5 [5.0] => 5.0 [5.5] => 5.5 [6.0] => 6.0 )

$item: Array ( [value] => 4 [_error_element] => field_sp_skill][0][value )

Is there some different way I should be setting allowed values for a decimal field such that they work, or is it just a number.module bug that needs to be fixed?