crashes on some lookups

[balboah]

Hello,

While using coredns 1.7.0 with the config:

    .:53 {
        debug
        health {
           lameduck 5s
        }
        ready
        prometheus :9153
        unbound

        cache

        loop
        reload
    }

And querying for ipleak.net, I get a crash:

panic: runtime error: index out of range [0] with length 0

goroutine 799 [running]:
github.com/coredns/coredns/plugin/cache.key(0xc00003e190, 0xb, 0xc0003a3a70, 0x0, 0x0, 0x0, 0xc0002a4300)
	/src/coredns-1.7.0/plugin/cache/cache.go:78 +0xca
github.com/coredns/coredns/plugin/cache.(*ResponseWriter).WriteMsg(0xc0002aa300, 0xc0003a3a70, 0x1, 0x1)
	/src/coredns-1.7.0/plugin/cache/cache.go:162 +0x165
github.com/coredns/unbound.(*Unbound).ServeDNS(0xc00060e6e0, 0xdf6380, 0xc0002a4330, 0xdfdfe0, 0xc0002aa300, 0xc0002a8d80, 0xc000070bf0, 0x1, 0x1)
	/go/pkg/mod/github.com/coredns/unbound@v0.0.6/unbound.go:140 +0x4ca
github.com/coredns/coredns/plugin.NextOrFailure(0xd00513, 0x4, 0xdebe80, 0xc00060e6e0, 0xdf6380, 0xc0002a4330, 0xdfdfe0, 0xc0002aa300, 0xc0002a8d80, 0x0, ...)
	/src/coredns-1.7.0/plugin/plugin.go:80 +0xfb

If I remove the cache entry from the config, I get no crash but also an empty reply.
If I leave cache but replace unbond with forward, everything works fine

It seems to only happen on some cases, resolving google.com works fine with the above example config