Tenant Trampoline, who's liable?

[Jack Forester]

Guess I need to add a trampoline clause to my rental agreements. Our tenant setup a trampoline, without my knowledge. My guess is that it is a used trampoline, because it has no net or spring cover. 

Well, it just so happens that I was also changing insurance companies, and the insurance company sent out a representative to take pictures. They then contacted me about the trampoline, stating that it needs to have a net installed or be removed from the property. I relayed the same to the tenant, who said she was ordering one (as suspected--they are great tentants). 

My initial thought was, "It'd not my trampoline. I didn't provide or install it, so why would I be liable?"