diff --git a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_mesh_at_isolevel.h b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_mesh_at_isolevel.h
index 2c91a8ad5ef..eb52b781667 100644
--- a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_mesh_at_isolevel.h
+++ b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_mesh_at_isolevel.h
@@ -18,6 +18,8 @@
 #include <CGAL/boost/graph/Euler_operations.h>
 #include <CGAL/boost/graph/named_params_helper.h>
 
+#include <unordered_map>
+
 namespace CGAL {
 namespace Polygon_mesh_processing {