diff --git a/Envelope_2/include/CGAL/Env_default_diagram_1.h b/Envelope_2/include/CGAL/Env_default_diagram_1.h
index ef3de77b686..1a98c8e3aeb 100644
--- a/Envelope_2/include/CGAL/Env_default_diagram_1.h
+++ b/Envelope_2/include/CGAL/Env_default_diagram_1.h
@@ -21,6 +21,9 @@
 
 #include <list>
 
+#include <CGAL/basic.h>
+#include <CGAL/memory.h>
+
 CGAL_BEGIN_NAMESPACE
 
 /*! \class