diff --git a/Nef_3/include/CGAL/Nef_3/SNC_external_structure.h b/Nef_3/include/CGAL/Nef_3/SNC_external_structure.h
index 89e5d17b04d..50074dc6ce7 100644
--- a/Nef_3/include/CGAL/Nef_3/SNC_external_structure.h
+++ b/Nef_3/include/CGAL/Nef_3/SNC_external_structure.h
@@ -30,6 +30,7 @@
 #include <CGAL/Nef_3/SNC_simplify.h>
 #include <map>
 #include <list>
+#include <unordered_map>
 
 #undef CGAL_NEF_DEBUG
 #define CGAL_NEF_DEBUG 43