[ns] error when I run my program

[lobmar@voila.fr]

Hi ns-users,

When I run my programm I have this error : 
soupir:~> ns s.tcl
num_nodes is set 50
warning: Please use -channel as shown in tcl/ex/wireless-mitf.tcl
INITIALIZE THE LIST xListHead
Starting Simulation...
can't read "Node_(1)": no such element in array
   while executing
"return $Node_($id)"
   (procedure "_o3" line 3)
   (Simulator get-node-by-id line 3)
   invoked from within
"$self get-node-by-id [lindex $L 0]"
   (procedure "_o3" line 14)
   (Simulator compute-hier-routes line 14)
   invoked from within
"$self compute-hier-routes "
   invoked from within
"if [Simulator hier-addr?] {
$self compute-hier-routes
} else {
$self compute-flat-routes
}"
   (procedure "_o3" line 2)
   (Simulator compute-routes line 2)
   invoked from within
"[Simulator instance] compute-routes"
   (procedure "Agent/rtProto/Static" line 2)
   (Agent/rtProto/Static init-all line 2)
   invoked from within
"Agent/rtProto/Static init-all"
   invoked from within
"if [info exists rtprotos_] {
foreach proto [array names rtprotos_] {
eval Agent/rtProto/$proto init-all $rtprotos_($proto)
}
} else {
Agent/rtProto/St..."
   (procedure "_o1347" line 3)
   (RouteLogic configure line 3)
   invoked from within
"[$self get-routelogic] configure"
   (procedure "_o3" line 5)
   (Simulator run line 5)
   invoked from within
"$ns_ run"
   (file "s.tcl" line 222)
----------------------------------------------------------------
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can any one help me please?

Thanks