dynamic update is not working for signed zone

[rams]

Hi,
When i do a dynamic update using nsupdate, i am unable to add record into
signed zone.
steps followed:
[root at stulcqacustbind2 muktha]# nsupdate
> server <server ip>
> update add net.rameshnu.sun. 86400 IN A 1.2.3.4
> send
update failed: SERVFAIL
>

Bind log:
25-Apr-2011 12:43:22.166 update: info: client ip#47830: updating zone
'net.rameshnu.sun/IN': adding an RR at 'net.rameshnu.sun' A
25-Apr-2011 12:43:22.167 update: error: client ip#47830: updating zone
'net.rameshnu.sun/IN': found no private keys, unable to generate any
signatures
25-Apr-2011 12:43:22.167 update: error: client ip#47830: updating zone
'net.rameshnu.sun/IN': RRSIG/NSEC/NSEC3 update failed: not found

Please clarify me.

Thanks & Regards,
Ramesh
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