Forum: FXRuby FXDirDialog.new Error Message

I want a dialog box to choose a directory.  I'm running the following
code

require 'fox16'
include Fox

dialog = FXDirDialog.new(self, "Choose Directory")

and am getting an error that says
"No matching function for overloaded 'new_FXDirDialog (ArgumentError)'"

Am I missing something, or typing something wrong?

Thanks,

Alex