diff --git a/Classification/include/CGAL/Classification/Label_set.h b/Classification/include/CGAL/Classification/Label_set.h
index d3e84acb4c9..cd42b34b909 100644
--- a/Classification/include/CGAL/Classification/Label_set.h
+++ b/Classification/include/CGAL/Classification/Label_set.h
@@ -19,6 +19,7 @@
 #include <CGAL/Random.h>
 
 #include <vector>
+#include <unordered_map>
 
 namespace CGAL {