TCL: syntax error in expression with "*"

I'm using tcl scripts in ns2 ( network simulator) through cygwin. It works fine , however, I downloaded an example when i run it , I got the following syntax error:

syntax error in expression with " *2" : unexpected operator *
while executing :
"expr $bw *2"
invoked from within:
"$ns duplex-link $n1 $n3 [expr  $bw *2] Mb [expr  $rtt_1 /2-1 ]ms DropTail"

I tried to comment this line to see if the error from something else, I got the error whenever there is a line with "*"

Any suggestions??:confused:

Does +2 work?
Is $bw really a number?