diff --git a/include/xo/unit/Quantity_iostream.hpp b/include/xo/unit/Quantity_iostream.hpp
index c1f6a717..27d5c458 100644
--- a/include/xo/unit/Quantity_iostream.hpp
+++ b/include/xo/unit/Quantity_iostream.hpp
@@ -6,6 +6,7 @@
 #pragma once
 
 #include "Quantity.hpp"
+#include "natural_unit_iostream.hpp"
 #include <iostream>
 
 namespace xo {
diff --git a/include/xo/unit/natural_unit_iostream.hpp b/include/xo/unit/natural_unit_iostream.hpp
index 99d3dd4d..aac612e3 100644
--- a/include/xo/unit/natural_unit_iostream.hpp
+++ b/include/xo/unit/natural_unit_iostream.hpp
@@ -6,6 +6,7 @@
 #pragma once
 
 #include "natural_unit.hpp"
+#include "bpu_iostream.hpp"
 #include <iostream>
 
 namespace xo {