diff --git a/Triangulation_2/include/CGAL/Constrained_triangulation_2.h b/Triangulation_2/include/CGAL/Constrained_triangulation_2.h
index 4441ca2e20d4..d77322d74378 100644
--- a/Triangulation_2/include/CGAL/Constrained_triangulation_2.h
+++ b/Triangulation_2/include/CGAL/Constrained_triangulation_2.h
@@ -31,6 +31,7 @@
 #include <CGAL/squared_distance_2.h>
 #include <CGAL/tags.h>
 #include <CGAL/Kernel_23/internal/Has_boolean_tags.h>
+#include <CGAL/NT_converter.h>
 
 #include <boost/mpl/has_xxx.hpp>
 #include <boost/iterator/filter_iterator.hpp>