Each of us has a Spirit Guide and Spirit Guardian (Guardian Angel) whether we know it or not. The Guardian acts as a gate keeper for a medium, lining up any spirits that they will contact and making sure that they have semi-good intentions for those the medium is interacting on behalf of. Not every medium is aware of this or will mention it.
This gatekeeper aspect will make sure that it is very unlikely that you will have something untoward happening to you.
If you're worried about it, then you can shield yourself from bad spirits by picturing a large white column of light coming from the heavens and enveloping around you in an egg of white, pure light and knowing that you are safe. Then stop thinking about that and go into the medium's shop.