This error you usually find while you try to check your dns server records with nslookup, if you hit  nslookup on cmd.

In most cases it happens because of incomplete ending of PTR record of BIND server. So first open reverse lookup zone and check it. if you mis  "." followed by name of your dns server in PTR record, simple do it and reload the zone,

$TTL 1D
@       IN SOA  ns.geekinlinux.com. root.ns.geekinlinux.com. (
                                        0       ; serial
                                        1D      ; refresh
                                        1H      ; retry
                                        1W      ; expire
                                        3H )    ; minimum

@       IN      NS      ns.geekinlinux.com. 
ns      IN      A       192.168.20.121
121     IN      PTR     ns.

that's it.