Querying for non existant reverse edge fails

I am using a tree structure with random depth and reverse edges to parents. If I try to use reverse edge where it doesnt exist, it fails, is there a way to avoid that? @filter exits or something similar?
func: uid(0xaaaa) {
test0 : ~parents {
uid
test1 : ~parents {
uid
}
}
}

What is the response you get?