[ns] attach agent to mobile node

hi,
I'm simulating an ad hoc net, I got the following error message when
executing my tcl file:

num_nodes is set 8
INITIALIZE THE LIST xListHead
wrong # args: should be "_o3 self class proc node agent"
(Simulator attach-agent line 1)
invoked from within
"$ns attach-agent $n(1) $tcp
(file "xxx.tcl" line 144)

here is some of the code:

set tcp [new Agent/TCP/Newreno]
$tcp set class_ 2
$tcp set window_ 2000

$ns attach-agent $n(1) $tcp #line 144

can anyone help me to fix this error or at least to understand the cause of
it
thanks