Combo控件引用ItemData()时出错。

Combo控件引用ItemData()时出错。请求各位解答
'两个Combo控件，一个text控件。网上找不到答案，才求助各位

Private Sub Combo1_Click()
Print Combo1.ItemData(Combo1.ListIndex) '引用正常
Print Combo2.ItemData(Combo2.ListIndex) '错误！！提示无效属性数组索引。 为什么Combo1可以，Combo2不可以？？
Text1.Text = Combo1.ItemData(Combo1.ListIndex) + Combo2.ItemData(Combo2.ListIndex) '单击选择Combo1时计算和并用Text1控件显示。出错！
End Sub

Private Sub Form_Load()
Combo1.AddItem "a": Combo1.ItemData(0) = 100
Combo1.AddItem "b": Combo1.ItemData(1) = 200
Combo1.AddItem "c": Combo1.ItemData(2) = 300
Combo1.ListIndex = 0

Combo2.AddItem "a": Combo2.ItemData(0) = 400
Combo2.AddItem "b": Combo2.ItemData(1) = 500
Combo2.AddItem "c": Combo2.ItemData(2) = 600
Combo2.ListIndex = 1

Show
Print Combo1.ItemData(Combo1.ListIndex) '此处引用ItemData()值正常
Print Combo2.ItemData(Combo2.ListIndex)
End Sub

Combo2里面添加些条目就好了