Expoo traffic??

[CWen56 <n2688420@dec4000.cc.ncku.edu.tw>]

Hi!
    I am a new-hand using NS-2. I am trying to do TCP simulation, but at
first I try a UDP simulation and got the following error message. I
can't realize what's wrong. But if i took off Expoo traffic, everything
would be fine. May soneone give me a clue how to realize the following
error message?
thanks
regards

################################
    (_o80 cmd line 1)
    invoked from within
"_o80 cmd attach-traffic _o82"
    invoked from within
"catch "$self cmd $args" ret"
    (procedure "_o80" line 2)
    (SplitObject unknown line 2)
    invoked from within
"$udp attach-traffic $traffic"
    (file "tcp.tcl" line 57)

Source Code:
############# Set up UDP agent ##################
set udp [new Agent/UDP]
$ns attach-agent $n1 $udp
#set cbr [new Application/Traffic/CBR]
#$cbr set packetSize_ 64
#$cbr set rate_ 64Kb
#$cbr attach-agent $udp
$udp set class_ 0
$udp set fid_ 0

#Create an Expoo traffic agent and set its configuration parameters
set traffic [new Traffic/Expoo]
$traffic set packetSize_ 512B
$traffic set burst_time_ 1.5s
$traffic set idle_time_ 0.5s
$traffic set rate_ 1000KB
$udp attach-traffic $traffic

set null [new Agent/Null]
$ns attach-agent $n3 $null

$ns connect $udp $null