diff --git a/include/xo/unit/unit.hpp b/include/xo/unit/unit.hpp
index 56157c94..d93ead8d 100644
--- a/include/xo/unit/unit.hpp
+++ b/include/xo/unit/unit.hpp
@@ -2,6 +2,7 @@
 
 #pragma once
 
+#include "unit_concept.hpp"
 #include "dimension_impl.hpp"
 
 namespace xo {