diff --git a/include/boost/program_options/detail/value_semantic.hpp b/include/boost/program_options/detail/value_semantic.hpp
index 80e292510..27b7c241b 100644
--- a/include/boost/program_options/detail/value_semantic.hpp
+++ b/include/boost/program_options/detail/value_semantic.hpp
@@ -17,13 +17,15 @@ namespace boost { template<class T> class optional; }
 
 namespace boost { namespace program_options { 
 
-    extern BOOST_PROGRAM_OPTIONS_DECL std::string arg;
+    namespace detail {
+        extern BOOST_PROGRAM_OPTIONS_DECL std::string arg;
+    }
     
     template<class T, class charT>
     std::string
     typed_value<T, charT>::name() const
     {
-        std::string const& var = (m_value_name.empty() ? arg : m_value_name);
+        std::string const& var = (m_value_name.empty() ? detail::arg : m_value_name);
         if (!m_implicit_value.empty() && !m_implicit_value_as_text.empty()) {
             std::string msg = "[=" + var + "(=" + m_implicit_value_as_text + ")]";
             if (!m_default_value.empty() && !m_default_value_as_text.empty())
diff --git a/src/value_semantic.cpp b/src/value_semantic.cpp
index de7b2ace8..c4feb688e 100644
--- a/src/value_semantic.cpp
+++ b/src/value_semantic.cpp
@@ -84,12 +84,14 @@ namespace boost { namespace program_options {
     }
 #endif
 
-    BOOST_PROGRAM_OPTIONS_DECL std::string arg("arg");
+    namespace detail {
+        BOOST_PROGRAM_OPTIONS_DECL std::string arg("arg");
+    }
 
     std::string
     untyped_value::name() const
     {
-        return arg;
+        return detail::arg;
     }
     
     unsigned