diff --git a/Filtered_kernel/include/CGAL/Lazy.h b/Filtered_kernel/include/CGAL/Lazy.h
index 204a2bf70cd..13412828169 100644
--- a/Filtered_kernel/include/CGAL/Lazy.h
+++ b/Filtered_kernel/include/CGAL/Lazy.h
@@ -37,6 +37,9 @@
 
 #include <boost/optional.hpp>
 #include <boost/variant.hpp>
+#ifdef CGAL_LAZY_KERNEL_DEBUG
+#  include <boost/optional/optional_io.hpp>
+#endif
 
 #include <boost/mpl/has_xxx.hpp>