diff --git a/lemon/bits/edge_set_extender.h b/lemon/bits/edge_set_extender.h
--- a/lemon/bits/edge_set_extender.h
+++ b/lemon/bits/edge_set_extender.h
@@ -280,6 +280,8 @@
 
     typedef EdgeSetExtender Graph;
 
+    typedef True UndirectedTag;
+
     typedef typename Parent::Node Node;
     typedef typename Parent::Arc Arc;
     typedef typename Parent::Edge Edge;
diff --git a/lemon/bits/graph_adaptor_extender.h b/lemon/bits/graph_adaptor_extender.h
--- a/lemon/bits/graph_adaptor_extender.h
+++ b/lemon/bits/graph_adaptor_extender.h
@@ -181,6 +181,8 @@
     typedef _Graph Graph;
     typedef GraphAdaptorExtender Adaptor;
 
+    typedef True UndirectedTag;
+
     typedef typename Parent::Node Node;
     typedef typename Parent::Arc Arc;
     typedef typename Parent::Edge Edge;