diff --git a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/Smoothing/curvature_flow_impl.h b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/Smoothing/curvature_flow_impl.h
index a201c6e8dde..0c9e2d9fa0e 100644
--- a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/Smoothing/curvature_flow_impl.h
+++ b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/Smoothing/curvature_flow_impl.h
@@ -35,6 +35,7 @@
 #include <iostream>
 #include <utility>
 #include <vector>
+#include <unordered_map>
 
 namespace CGAL {
 namespace Polygon_mesh_processing {