Spring 3.0.6 taglib - form:checkbox not creating hidden field

I am using Spring 3.0.6 and their tag library. I am using the form:checkbox tag. From what I read and researched, it is supposed to create a hidden field with the same name and a leading _ character. This tells spring whether the checkbox was checked or not so that it will properly set my model attribute.

The problem is that I am not seeing a hidden field created at all for this. I thought it might be my