ACCESS::session sid fails some times

Hi

I have something like this

if { ( [ACCESS::session exists] ) } {

  set un_ec_sid [ACCESS::session sid]

  some other stuff

}

and I get in /var/log/ltm

session ID lookup failed - Expired (line 1) invoked from within "ACCESS::session sid"

not sure how that can fail the exists statemant confirms its valid but then suddenly it dies ?

also is the SID the same as MRHSESSION Cookie ?