Error while setting the Tx and Rx power

["N.V.Balaji" <csa98011@cse.iitd.ernet.in>]

Hi all,
When I Try to set the receiving and Transmission powers of a mobile I get
the following error:

can't read "netif(0)": no such variable
    while executing
"$netif(0) setRxPower $val"
    (procedure "_o16" line 3)
    (Node/MobileNode setPr line 3)
    invoked from within
"$node_($i) setPr 2.85"


Could you please point out the error ?

Thanks in Advance
Balaji N V