diff --git a/traitsui/tree_node.py b/traitsui/tree_node.py
index f29138a58..c05941587 100644
--- a/traitsui/tree_node.py
+++ b/traitsui/tree_node.py
@@ -246,7 +246,7 @@ def append_child(self, object, child):
     def insert_child(self, object, index, child):
         """ Inserts a child into the object's children.
         """
-        children = self.get_children()
+        children = self.get_children(object)
         children[index:index] = [child]
 
     #-------------------------------------------------------------------------