diff --git a/include/xo/unit/scaled_unit.hpp b/include/xo/unit/scaled_unit.hpp
index 166acc6e..ac6fd60a 100644
--- a/include/xo/unit/scaled_unit.hpp
+++ b/include/xo/unit/scaled_unit.hpp
@@ -31,7 +31,7 @@ namespace xo {
 
             /** @brief true iff scaled unit can be faithfully represented by a @ref natural_unit **/
             constexpr bool is_natural() const {
-                return (outer_scale_factor_ = OuterScale(1) && (outer_scale_sq_ == 1.0));
+                return (outer_scale_factor_ == OuterScale(1) && (outer_scale_sq_ == 1.0));
             }
 
         public: /* need public members so that a scaled_unit instance can be a non-type template parameter (a structural type) */