diff --git a/include/boost/math/special_functions/relative_difference.hpp b/include/boost/math/special_functions/relative_difference.hpp
index 465184ae7..baf231b03 100644
--- a/include/boost/math/special_functions/relative_difference.hpp
+++ b/include/boost/math/special_functions/relative_difference.hpp
@@ -71,7 +71,7 @@ namespace boost{
 
 #if defined(macintosh) || defined(__APPLE__) || defined(__APPLE_CC__)
       template <>
-      inline double relative_difference<double>(double a, double b)
+      inline typename boost::math::tools::promote_args<double, double>::type relative_difference(const double& arg_a, const double& arg_b)
       {
          BOOST_MATH_STD_USING
          //