diff --git a/droidbot/device_state.py b/droidbot/device_state.py
index 5cb85555..932886d6 100644
--- a/droidbot/device_state.py
+++ b/droidbot/device_state.py
@@ -386,7 +386,7 @@ def get_all_children(self, view_dict):
         children = set(children)
         for child in children:
             children_of_child = self.get_all_children(self.views[child])
-            children.union(children_of_child)
+            children = children.union(children_of_child)
         return children
 
     def get_app_activity_depth(self, app):