Using condition builder field control with depending fields

I've got a form within a module where I need two fields to be interdependent - if one is filled in, the other should be disabled, and vice versa. However, one of the fields is using the condition builder form control which is causing the following error when the @has depending fields@ checkbox is checked:[Error loading the FormControl 'ConditionBuilder'] Control with ID 'txtMacro' being registered through RegisterAsyncPostBackControl or RegisterPostBackControl must implement either INamingContainer, IPostBackDataHandler, or IPostBackEventHandler. Has anyone encountered this, and know how it might be solved? I've tried making the condition builder implement IPostBackDataHandler, which gets rid of that error but doesn't actually work to do what I want it to do (it never hits the implemented methods).