diff --git a/maths.hpp b/maths.hpp
index ce8433ce..1d80b106 100644
--- a/maths.hpp
+++ b/maths.hpp
@@ -38,6 +38,7 @@
 namespace cruft {
     ///////////////////////////////////////////////////////////////////////////
     template <typename T>
+    constexpr
     std::enable_if_t<std::is_arithmetic_v<T>, T>
     abs [[gnu::const]] (T t)
     {