diff --git a/robowaiter/behavior_lib/cond/HasSubTask.py b/robowaiter/behavior_lib/cond/HasSubTask.py
index 3bbc52f..571c8c0 100644
--- a/robowaiter/behavior_lib/cond/HasSubTask.py
+++ b/robowaiter/behavior_lib/cond/HasSubTask.py
@@ -8,7 +8,7 @@ class HasSubTask(Cond):
 
     def _update(self) -> ptree.common.Status:
         # if self.scene.status?
-        if self.scene.sub_task_seq.children == []:
+        if not self.scene.sub_task_seq or not self.scene.sub_task_seq.children:
             return ptree.common.Status.FAILURE
         else:
             return ptree.common.Status.SUCCESS
diff --git a/robowaiter/robot/robot.py b/robowaiter/robot/robot.py
index 6d7b275..185c7d7 100644
--- a/robowaiter/robot/robot.py
+++ b/robowaiter/robot/robot.py
@@ -70,7 +70,11 @@ class Robot(object):
         del_sub_tree.set_scene(self.scene)
         seq.add_child(del_sub_tree)
 
-        self.scene.sub_task_seq.add_child(seq)
+        if self.scene.sub_task_seq:
+            self.scene.sub_task_seq.add_child(seq)
+        else:
+            print('Warning: have none sub task sequence')
+            self.scene.sub_task_seq = seq
         print("当前行为树为：")
         print_tree_from_root(self.bt.root)