Slotmapping doesn't work

I have made a formAction and am trying to use slot_mappings but can’t seem to get it work. This is what my slot_mapping looks like:

    def slot_mappings(self) -> Dict[Text, Union[Dict, List[Dict]]]:

        return {
            "invite_more_bool": [
                self.from_intent(intent="affirm", value=True),
                self.from_intent(intent="deny", value=False),

However I also have a validate function and once we enter validate() the slot “invite_more_bool” is still not set but has the value None. Why is this? Thanks!