Panic on non existence permission?

[nmsobri]

Let say i have this sorts of permissions:

permissions["view_sales"] = gorbac.NewStdPermission("view_sales")
permissions["edit_sales"] = gorbac.NewStdPermission("edit_sales")

if rbac.IsGranted("cashier", permissions["delete_sales"], nil) {
    fmt.Println("Permission delete_sales::YES")
}

gorbac will simply throw a panic on me if delete_sales is not a valid / non existence permission.
Is there a way to simply return false instead?