diff --git a/include/boost/optional/optional.hpp b/include/boost/optional/optional.hpp
index f059e2e..9def94e 100644
--- a/include/boost/optional/optional.hpp
+++ b/include/boost/optional/optional.hpp
@@ -1271,9 +1271,10 @@ get_pointer ( optional<T>& opt )
 // The following declaration prevents a bug where operator safe-bool is used upon streaming optional object if you forget the IO header.
 template<class CharType, class CharTrait>
 std::basic_ostream<CharType, CharTrait>&
-operator<<(std::basic_ostream<CharType, CharTrait>&, optional_detail::optional_tag const&)
+operator<<(std::basic_ostream<CharType, CharTrait>& os, optional_detail::optional_tag const&)
 {
-  BOOST_STATIC_ASSERT_MSG(sizeof(CharType) == 0, "If you want to output boost::optional, include header <boost/optional/optional_io.hpp>"); 
+  BOOST_STATIC_ASSERT_MSG(sizeof(CharType) == 0, "If you want to output boost::optional, include header <boost/optional/optional_io.hpp>");
+  return os;  
 }
 
 // optional's relational operators ( ==, !=, <, >, <=, >= ) have deep-semantics (compare values).