Script to generate random string not working

I'm trying to generate random string with white spaces but it is not working :

/rs {
  %i=1
  %r=$rand(1,50)
  %s=$rand(a,z)
  while(%i <= %r) {
    %i=%i+1
    %s=%s $+ $rand(a,z)
    if(1 == rand(1,4) %s=%s $+ $chr(32)
  }
  echo %s
}

Returns :

WHILE(%I Unknown command

Any ideas ?