[ns] How to attach flow monitor to FQ link

[Kenny Pounds <hemin99@chinaren.com>]

Hi, all,
I want to monitor the flows' queueing delay.
And I tried the method similiar to CBQ delay trace.
However, I can't achieve the same goal.

For example, 
	........
	$ns simplex-link $n2 $n3 1.5Mb 10ms FQ
	.........
	set link3 [$ns link $n2 $n3]
	set fmon [$ns makeflowmon Fid]
	$ns attach-fmon $link3 $fmon
	set fcl [$fmon classifier]
	set flow [$fcl lookup auto 0 0 $i]
	set dsamp [$flow get-delay-samples]
	set meandelay($i) [$dsamp mean]
	.........
But I got the error as follows:

ns: cdumpdel; finish Output: _o40: unable to dispatch method attach-monitors
    while executing
"$lnk attach-monitors $isnoop $osnoop $dsnoop $fm"
    (procedure "_o4" line 5)
    (Simulator attach-fmon line 5)
    invoked from within
"$ns attach-fmon $link3 $fmon"
    (procedure "cdumpdel" line 7)
    invoked from within
"cdumpdel"            

However, if I change FQ to DropTail, this code will be OK. Who can tell me
what's the trick?