diff --git a/Number_types/test/Number_types/checked_NT.h b/Number_types/test/Number_types/checked_NT.h
index 26b4a913e08..4de367c19d2 100644
--- a/Number_types/test/Number_types/checked_NT.h
+++ b/Number_types/test/Number_types/checked_NT.h
@@ -6,6 +6,7 @@
 #include <CGAL/Real_embeddable_traits.h>
 #include <CGAL/number_utils.h>
 #include <CGAL/enum.h>
+#include <CGAL/is_convertible.h>
 #include <boost/utility/enable_if.hpp>
 
 namespace CGAL {