[ns] error in running the simulation

Dear Ns-Users,

I am running a tcl file under ns2.34. Every things seems to go well with me, but at the end of running the simulation it gives me the following:

ns: _o3 prepare-to-stop: can't read "lagent": no such variable
    while executing
"foreach i $lagent {
$i stop
}"
    (procedure "_o3" line 3)
    (Simulator prepare-to-stop line 3)
    invoked from within
"_o3 prepare-to-stop"

Could any body help please?

 Thank you in advance.

 
Tamer Al-Jahamneh