DNS forwarding issue in bind9

[manfly000]

I configure slave zone(rog2.example.com) on DNS server(bind9)
When I query w1.rog2.example.com by using dig tool, it is failed. the slave zone configuration as follows:
zone "rog2.rchland.ibm.com"{
type slave;
file "DB.rog2.example.com.";
masters {
9.5.138.139;
};
forward only;
forwarders {9.5.138.132;};
}

Following is the failed logs:

16-Sep-2009 02:33:01.911 client: debug 3: client 9.123.136.86#3901: UDP request
16-Sep-2009 02:33:01.911 client: debug 5: client 9.123.136.86#3901: using view '_default'
16-Sep-2009 02:33:01.911 security: debug 3: client 9.123.136.86#3901: request is not signed
16-Sep-2009 02:33:01.911 security: debug 3: client 9.123.136.86#3901: recursion available
16-Sep-2009 02:33:01.911 client: debug 3: client 9.123.136.86#3901: query
16-Sep-2009 02:33:01.911 queries: info: client 9.123.136.86#3901: query: w1.rog2.example.com IN A +
16-Sep-2009 02:33:01.912 client: debug 10: client 9.123.136.86#3901: ns_client_attach: ref = 1
16-Sep-2009 02:33:01.912 client: debug 3: client 9.123.136.86#3901: error
16-Sep-2009 02:33:01.912 client: debug 3: client 9.123.136.86#3901: send
16-Sep-2009 02:33:01.912 client: debug 3: client 9.123.136.86#3901: sendto
16-Sep-2009 02:33:01.912 client: debug 3: client 9.123.136.86#3901: senddone
16-Sep-2009 02:33:01.912 client: debug 3: client 9.123.136.86#3901: next
16-Sep-2009 02:33:01.912 client: debug 10: client 9.123.136.86#3901: ns_client_detach: ref = 0
16-Sep-2009 02:33:01.912 client: debug 3: client 9.123.136.86#3901: endrequest
16-Sep-2009 02:33:01.912 client: debug 3: client @201b8f10: udprecv

but when I change the zone to forward zone as follows:
When I query w1.rog2.example.com by using dig tool, it is successful to get the A record in forwarder server 9.5.138.132.
zone "rog2.rchland.ibm.com"{
type forward;
forward only;
forwarders {9.5.138.132;};
}

In slave zone, It seems that the forwarder has no effect, because when I delete the forward statements as follows, still the same failed logs as above.

zone "rog2.rchland.ibm.com"{
type slave;
file "DB.rog2.example.com.";
masters {
9.5.138.139;
};
}

I don't know whether it is bind9's issue?