for example i want to read the  value of  sip-header ' CSeq'   in lua(that is configure for mod_xml in /usr/loca/freeswitch/conf/autoload/lia.xml.conf), must to issue as follow:

local var = session:getVariable("sip_CSeq")

but the error occure:

 attempt to index global 'session' (a nil value)