diff --git a/lib/ridl/runner.rb b/lib/ridl/runner.rb
index 65e15e7..e099684 100644
--- a/lib/ridl/runner.rb
+++ b/lib/ridl/runner.rb
@@ -367,7 +367,7 @@ def init_optparser
         self.options[:debug] = true
       }
       opts.on('--idl-version=VERSION', Integer,
-              'Set IDL version',
+              'Set IDL version (3|4)',
               'Default: 3') { |v|
         self.options[:idlversion] = v
       }