我写了如下代码，动作是在组合框ComBo1选择一个列表中的值以后， 一次作为查询条件打开Recordset对象Rs。但是程序运行到Item.ListIndex报错：编译错误：必选参数。
这是什么意思？究竟错在哪里？请高手指教。

Public Cnn As New ADODB.Connection
Dim Rs As New ADODB.Recordset
Dim Constring As String
Dim Lv As ListItem

Private Sub Combo1_Click(Index As Integer)
Rs.CursorLocation = adUseClient
Rs.Open "SELECT BloodSerum.BloodID, BloodSerumQuery.AnimalCode, BloodSerumQuery.BleededDate, BloodSerumQuery.Remark, BloodSerumQuery.CatalogNo, BloodSerumQuery.LightChain FROM BloodSerumQuery Where BloodSerumQuery.LightChain Like " & Combo1.Item.List(Combo1.Item.ListIndex), Cnn, adOpenForwardOnly, adLockReadOnly
......
End Sub