diff --git a/test/concepts/test_bug_11988.cpp b/test/concepts/test_bug_11988.cpp
index 2d921024..840ba9af 100644
--- a/test/concepts/test_bug_11988.cpp
+++ b/test/concepts/test_bug_11988.cpp
@@ -9,16 +9,19 @@
 *
 */
 
-#include <boost/regex.hpp>
 
 #ifndef BOOST_NO_CXX11_CHAR32_T
 
+#include <cstddef>
+
 namespace boost {
 
-   std::size_t hash_value(char32_t c) { return c; }
+   std::size_t hash_value(char32_t const& c) { return c; }
 
 }
 
+#include <boost/regex.hpp>
+
 struct char32_traits
 {
    typedef char32_t char_type;