[Exim] dbdns error

I'm using dbdns to do a lookup of the NS record of IP addresses and I
have a proble. When there is no NS record I get this error. What I want
is to return an empty string. How do I fix this?

headers command: lookup of "ns=136.79.149.in-addr.arpa" gave DEFER:


2003-12-22 07:39:22 1AYS9Q-0007uH-KY Error in system filter: failed to
expand "X-Sender-Nameserver: ${sg{${lookup
dnsdb{ns=${sg{$sender_host_address}{\N([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})\N}{\N$3.$2.$1.in-addr.arpa\N}}}}
${lookup
dnsdb{ns=${sg{$sender_host_address}{\N([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})\N}{\N$2.$1.in-addr.arpa\N}}}}}{\n}{
}}" in headers command: lookup of "ns=136.79.149.in-addr.arpa" gave DEFER: