Why? Don't you know what you don't believe in?
The thing with "disbelief" is that it can mean two things:
1. Disbelief can be when a person is convinced something is false, and they know that something. This is disbelief in effect and in motivation.
and
2. Disbelief can be when a person doesn't know something in the first place - for them, neither the notion of "belief" nor "disbelief" apply in that instance. This is disbelief in effect, but not in motivation.