diff --git a/src/include/units/quantity.h b/src/include/units/quantity.h
index 7efdc723..57236ab0 100644
--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -41,8 +41,8 @@ concept floating_point_ = // exposition only
 
 template<typename From, typename To>
 concept safe_convertible_to_ = // exposition only
-    !(Quantity<From>) &&
-    !(Quantity<To>) &&
+    (!Quantity<From>) &&
+    (!Quantity<To>) &&
     std::convertible_to<From, To> &&
     (floating_point_<To> || (!floating_point_<From>));