Integer field + checkboxes/radio widget: option with key = 0 lose his value when there are validation errors in the node form

Hi!

The node has the field of type "Integer" with "Checkboxes/radio" widget. The field is multivalued, so it renders in the node editing form like a few checkboxes (see screenshots).

Edit the node... Set the all checkboxes, make some field(s) nonvalid and hit "Save" button. The value with key = 0 become unchecked (see screenshots). The keys <0 and >0, the other widget (Select list) - work fine. Only key = 0 causes the problem.

I use clean Drupal 6.22 and CCK 6.x-2.9. The only enabled modules: Content, Number, Option Widgets (+ required core modules).