Failed to create master zone : Invalid master IP

I am running 2 systems both are ubuntu 10.4 running virtualmin on 2 seprate sumbets. I am usieng both of them for DNS servers one is the master server the other is the slave server. i followed the directions here ( for setting it up and it wokred just fine

the servers are configured with both ipv4 and ipv6 addresses as exampled

MASTER (yes its natted)

SLAVE (yes its natted)

in the module config for bind on the master server under cluster slave servers i have
Default master server IP for remote slave zones set to 2001:470:1564:1756::73

  Additional master IP addresses for slave zones    set to   2001:470:1564:1756:153f:fea4:887a:ffeb

when i try and add a zone on the master server i get the error
Failed to create master zone : Invalid master IP 2001:470:1564:1756::73

my servers are authoritive for my ip6 networks any idea what i am doing wrong.