diff --git a/include/xo/unit/quantity.hpp b/include/xo/unit/quantity.hpp
index 0760d428..178cf2e9 100644
--- a/include/xo/unit/quantity.hpp
+++ b/include/xo/unit/quantity.hpp
@@ -508,7 +508,7 @@ namespace xo {
                   && Q1::always_constexpr_unit
                   && Q2::always_constexpr_unit)
         constexpr auto
-        with_units_from(const Q1 & x, const Q2 & y)
+        with_units_from(const Q1 & x, const Q2 & /*y*/)
         {
             return x.template rescale_ext<Unit>();
         }