diff --git a/Installation/include/CGAL/stddef.h b/Installation/include/CGAL/stddef.h
new file mode 100644
index 00000000000..03d4022a812
--- /dev/null
+++ b/Installation/include/CGAL/stddef.h
@@ -0,0 +1 @@
+#error You have added .../path/to/CGAL-x.y/include/CGAL to the INCLUDE path. You should use .../path/to/CGAL-x.y/include instead.