I config ims_diameter_server as below: ``` #!ifdef WITH_DIAMETER_SERVER # IMS_DIAMETER_SERVER event_route[diameter:request]{ xlog("L_INFO","ims Got diameter message"); # diameter_request("hss.ims.mnc001.mcc001.3gppnetwork.org", $diameter_application, $diameter_command, $diameter_request); diameter_request_async("hss.ims.mnc001.mcc001.3gppnetwork.org", $diameter_application, $diameter_command, $diameter_request); xlog("L_INFO","ims Forwarded Diameter Request"); } event_route[diameter:response] { xlog("L_INFO","ims Reply to Diameter request is $diameter_response\n"); } #!endif ``` And got a response from hss:  but got <null> response in log:
90(124) INFO: <script>: ims Got diameter message90(124) DEBUG: ims_diameter_server [avp_helper.c:59]: avp2json(): AVP((nil) < 0x7f44c2ad16f0 >0x7f44c2ad0c08);code=263,flags=40; 89(123) INFO: <script>: ims Reply to Diameter request is <null>
$diameter_response is <null>, why, how to fix it?