How do I keep a bed that is on wheels from rolling away?

Ok, jellies – this is driving me quite insane. I am staying with friends, and they have me in a guest room on a bed that has wheels. I am grateful for their hospitality, don’t get me wrong… but I am very much a “sit up and read in bed before going to sleep” kind of person, and whenever I try to do that, the bed starts rolling away from the headboard, to which it is not attached. Does anyone know of a solution to this? It clearly can’t be a permanent one.