[db-wg] Since when does one need 3 nameservers for inverse?

In the "Reverse Delegation How To": http://www.ripe.net/rs/reverse/reverse_howto.html it states: "NS Servers Ensure you have at least two nameservers that are authoritative for the zone. The resolvable names of these NS servers should be in the NS resource records of the zone. The nameservers should be on different subnets." Yet when I submit a request I get: ***RDNS: (related to set) ERROR (20 points): At least 3 nameservers are required for each properly delegated zone. We found only 2 in your submission. In addition the reverse delegation checker at: http://www.ripe.net/cgi-bin/delcheck/delcheck2.cgi doesn't flag only 2 nameservers as an error. What gives? -Hank