Pan, use this one here:

on *:text:$($iif(!whois* iswm $strip($1),$1)):#:{
  var %2 = $strip($2)
  if ($nick isreg #) {
    msg # Sorry $nick $+ , you cannot use this command.
  }
  elseif (!%2) {
    msg # Syntax is !whois <nick to whois>.
  }
  else {
    set %WhoisResult #
    whois %2
  }
}