Slave root zone and forward zones with Bind

If you configured the DNS root zone as type slave, your Bind nameserver won't resolve forward zones beneath non existing TLDs, i.e. the following named.conf snippet won't work as one might expect:

zone "." {
        type slave;
        file "/etc/namedb/slave/root.slave";
        notify no;
        masters {

zone "testtest.intern" {
        type forward;
        forwarders {; };

testtest.intern won't resolve. In fact, won't even be queried.