diff --git a/include/tclap/Arg.h b/include/tclap/Arg.h
index da36513..59f3f42 100644
--- a/include/tclap/Arg.h
+++ b/include/tclap/Arg.h
@@ -263,7 +263,7 @@ class Arg
 		/**
 		 * Indicates whether the argument is required.
 		 */
-		bool isRequired() const;
+		virtual bool isRequired() const;
 
 		/**
 		 * Sets _required to true. This is used by the XorHandler.