diff --git a/maths.hpp b/maths.hpp
index 99622184..c4f31c7f 100644
--- a/maths.hpp
+++ b/maths.hpp
@@ -207,8 +207,8 @@ namespace cruft {
     ///------------------------------------------------------------------------
     /// Calculates the base-2 logarithm of an integer, rounding up to the next
     /// highest integer.
-    template <typename T>
-    T
+    template <std::integral T>
+    constexpr T
     log2up  (T val)
     {