diff --git a/src/core/include/units/magnitude.h b/src/core/include/units/magnitude.h
index 159282f9..c51285e2 100644
--- a/src/core/include/units/magnitude.h
+++ b/src/core/include/units/magnitude.h
@@ -525,7 +525,7 @@ constexpr auto denominator(Magnitude auto m) { return numerator(pow<-1>(m)); }
 
 // Implementation of conversion to ratio goes here, because it needs `numerator()` and `denominator()`.
 constexpr ratio as_ratio(Magnitude auto m)
-  requires(is_rational(decltype(m){}))
+  requires(is_rational(m))
 {
   return ratio{
     get_value<std::intmax_t>(numerator(m)),