[SR-Users] call_id in dialog module

hi all;i want to write "billing" project with  kamailio,i using python,postgresql, dialog module and falcon.my problem is in "call_id" section.my code(this section) is in python : """... if (method   ==  'INVITE'):             callid          =        req.get_param_as_list("call_id")             print(req.get_param_as_list("Ts"))             print("call id = %s " % callid) elif (method   ==  'ACK'):                        time_req    =       int(req.get_param_as_list("Ts")[0])             callid      =        req.get_param_as_list("call_id")[0]             print("user %s call to %s at time %d %            (from_user,to_user,time_req))...          """ and in kamailio.cfg: """http_query("http://127.0.0.1:5000/test?method=$rm&            call_id=$dlg(callid)&Ts=$Ts&fu=$fu&tu=$tu","$var(result)"); """" "callid" in ACK is ok but "callid' in invite is "null", where is my problem? thanks.