diff --git a/src/core/include/mp-units/framework/unit_concepts.h b/src/core/include/mp-units/framework/unit_concepts.h
index 94e430d0..9815db8a 100644
--- a/src/core/include/mp-units/framework/unit_concepts.h
+++ b/src/core/include/mp-units/framework/unit_concepts.h
@@ -190,7 +190,8 @@ concept UnitOf =
 
 namespace detail {
 
-[[nodiscard]] consteval auto have_same_canonical_reference_unit(Unit auto u1, Unit auto u2);
+template<Unit U1, Unit U2>
+[[nodiscard]] consteval auto have_same_canonical_reference_unit(U1 u1, U2 u2);
 
 }