[Grails] Why I'm getting this behavior with check box?

[John Todd]

Hey,
I have a boolean property in a domain class.
I create this line in create.gsp

<g:checkBox id="billable" name="billable" value="${fieldValue(bean: maintenanceInstance, field: 'billable')}"/> 

I checked the check box and submit the form, but I got this validation message (defined by me):

maintenance.billable.nullable=Billable should holds a number

Why I'm getting this?
Thanks for help and time.