Tried to set non existent slot 'None'

Hello! I use rasa shell --debug. Has this message after /restart.

ERROR rasa.core.trackers - Tried to set non existent slot ‘None’. Make sure you added all your slots to your domain file.

Could you help to explain what it means?

You try to set a Slot name ‘None’, which is not listed in your domain-File. There’s probably a bug in one of your actions, where it should obtain a slotname, but just gets the None-value.

Thanks for reply! But I didn’t set any slots, i just call /restart command