diff --git a/lib/redmine/menu_manager.rb b/lib/redmine/menu_manager.rb
index 996b1c5..0310fad 100644
--- a/lib/redmine/menu_manager.rb
+++ b/lib/redmine/menu_manager.rb
@@ -74,6 +74,7 @@ module TreeNodePatch
     # parent is set to be the receiver.  The child is added as the last child in
     # the current list of children for the receiver node.
     def add(child)
+      @childrenHash ||= {}
       raise "Child already added" if @childrenHash.has_key?(child.name)
 
       @childrenHash[child.name]  = child