diff --git a/Spatial_sorting/include/CGAL/Hilbert_sort_2.h b/Spatial_sorting/include/CGAL/Hilbert_sort_2.h
index 19998bb7998..003e0b06749 100644
--- a/Spatial_sorting/include/CGAL/Hilbert_sort_2.h
+++ b/Spatial_sorting/include/CGAL/Hilbert_sort_2.h
@@ -3,6 +3,8 @@
 
 #include <CGAL/basic.h>
 
+#include <functional>
+
 #include <CGAL/Hilbert_sort_base.h>
 
 CGAL_BEGIN_NAMESPACE
diff --git a/Spatial_sorting/include/CGAL/Hilbert_sort_3.h b/Spatial_sorting/include/CGAL/Hilbert_sort_3.h
index 3fb986f7802..6745b4a5d11 100644
--- a/Spatial_sorting/include/CGAL/Hilbert_sort_3.h
+++ b/Spatial_sorting/include/CGAL/Hilbert_sort_3.h
@@ -3,6 +3,8 @@
 
 #include <CGAL/basic.h>
 
+#include <functional>
+
 #include <CGAL/Hilbert_sort_base.h>
 
 CGAL_BEGIN_NAMESPACE