[ns] problem in running file 'simple-wireless.tcl'

[Maneesh Tewari <tmaneesh@cedt.iisc.ernet.in>]

hi ,

here whenever i'm trying to run the file
<ns-home-dir>/tcl/ex/simple-wireless.tcl

i'm getting following mesg.  and the output trace file 'simple.tr' is of
0
bytes.

>>>

Starting Simulation...
routes not yet computed
    (_o55 cmd line 1)
    invoked from within
"$self cmd lookup $nodeid $destid"
    (procedure "_o55" line 19)
    (RouteLogic lookup line 19)
    invoked from within
"$r lookup $i $j"
    (procedure "_o3" line 28)
    (Simulator compute-flat-routes line 28)
    invoked from within
"$self compute-flat-routes"
    (procedure "_o3" line 7)
    (Simulator compute-routes line 7)
    invoked from within
"[Simulator instance] compute-routes"
    (procedure "Agent/rtProto/Static" line 3)
    (Agent/rtProto/Static init-all line 3)
    invoked from within
"Agent/rtProto/Static init-all"
    (procedure "_o55" line 8)
    (RouteLogic configure line 8)
    invoked from within
"[$self get-routelogic] configure"
    (procedure "_o3" line 5)
    (Simulator run line 5)
    invoked from within
"$ns_ run"
    (file "simple-wireless.tcl" line 147)
>>>>

can anyone help me to get rid of this problem and   what it mean by

routes not yet computed
    (_o55 cmd line 1)
    invoked from within
"$self cmd lookup $nodeid $destid"


i'm using ns-allinone -2.1b6 on linux .

thanks in advance.

                                                      ---maneesh