diff --git a/maths.hpp b/maths.hpp
index 01309770..747f9f7f 100644
--- a/maths.hpp
+++ b/maths.hpp
@@ -115,7 +115,7 @@ namespace util {
         std::is_arithmetic<Tb>::value,
         bool
     >
-    exactly_equal (const Ta a, const Tb b)
+    exactly_equal (const Ta &a, const Tb &b)
     {
         return a == b;
     }