Dirty doesn't track integers changing from 0 to null

I've just noticed that when editing numeric fields, and changing the value from 0 to '', the alteration isn't saved. I think it's a bug in dirty...

>> l = Lookup.first

=> #<Lookup id: 1, field_name: "yn", option_number: 0, option_description: "No", created_by: 1, updated_by: 1, created_at: "2007-08-23 15:15:42", updated_at: "2007-08-23 15:15:42">

>> l.option_number = ''

=> ""

>> l.changed?

=> false

>> l.changes

=> {}