Quick rDNS Question

Trying to change the rDNS entry of a host. At the moment the IP is set to a custom entry but I want to change this to rhea.tokenbay.co.uk. I have an the necessary A record in the zone file.

When I try to update the record, it gives the following error:

An unexpected error was encountered while looking up the A record for hostname rhea.tokenbay.co.uk.. Please try again.

Querying 212.23.3.100 gives:

root@rhea:/var/spool/exim/log# nslookup rhea.tokenbay.co.uk 212.23.3.100
Server: 212.23.3.100
Address: 212.23.3.100#53

Non-authoritative answer:
Name: rhea.tokenbay.co.uk
Address: 217.155.123.100

Works OK here as I just changed mine about to check.

Did you remember to put the trailing period (.) at the end of your hostname?

I've just tried again now and it worked.

Not sure what happened last night, maybe the servers were on a tea break.