nxdomain not caching for configured reverse lookup

[sumsum 2000]

Hi,
I have the following  zone configuration for forwarding DNS query

                        view "default" IN {
                        max-cache-ttl 604800;
                        max-ncache-ttl 10800;

                        zone  "." IN  {
                                type forward;
                                forwarders {161.69.96.5;};
                                forward only;
                        };

                        zone "7.7.7.7.in-addr.arpa" IN {
                                type forward;
                                forwarders {10.212.24.11;};
                                forward only;
                        };
                        };

 when I do dig -x 9.9.9.9, which is not in the configured zone for DNS
which falls to the default DNS server 161.69.96.5, i get the resonses cached

 and when i do  dig -x 7.7.7.7, which is in the configured zone for DNS
10.212.24.11, i am not able to get the responses cached.

When i reverse the DNS entries for the default and the configured zones, i
get the same answer.

I would like to get the configured zones NXDOMAIN also cached, so that the
next time, the request comes, it should be served from the cache.

What am I missing?
Thanks
S
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