diff --git a/include/boost/system/detail/error_category.hpp b/include/boost/system/detail/error_category.hpp
index 07c0aa7..b649ef9 100644
--- a/include/boost/system/detail/error_category.hpp
+++ b/include/boost/system/detail/error_category.hpp
@@ -156,7 +156,7 @@ public:
 
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
-    operator std::error_category const & () const;
+    operator std::error_category const & () const BOOST_SYMBOL_VISIBLE;
 
 #endif
 };