Strange error

[Frank Chu <frankchu_ns@yahoo.com>]

This code generated an error:
  can't read "ftp(1)": no such element in array
      while executing
  "$ns at 1.0 "$ftp($i) start""

The code is:  (Note: MAX_TCP_PAIRS is 10)

 for {set i 1} {$i<=$MAX_TCP_PAIRS} {incr i 1} {
  set tcpsource(i) [new Agent/TCP/Reno]
  $ns attach-agent $tcpsourcenode(i) $tcpsource(i)
  set tcpsink(i) [new Agent/TCPSink]
  $ns attach-agent $tcpsinknode(i) $tcpsink(i)
  $ns connect $tcpsource(i) $tcpsink(i)
  set ftp(i) [new Application/FTP]
  $ftp(i) attach-agent $tcpsource(i)
  $ns at 1.0 "$ftp($i) start"  <-------- error appears
  $ns at 61.0 "$ftp($i) stop"
 }                           

very strange error ?

 Frank
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