html select控件selectedIndex无法赋值！

sqlconn.datac()
Dim sql As String = "select * from webadmin where id='" + Request("id") + "'"
Dim cmd As SqlCommand = New SqlCommand(sql, sqlconn.conn)
Dim rs As SqlDataReader
rs = cmd.ExecuteReader
If rs.Read Then
username.Text = rs.Item("username")
selladdress.Value = rs.Item("selladdress")
usertype.Items.FindByText(rs.Item("usertype")).Selected = True
worknum.Value = rs.Item("worknum")
manager.Value = rs.Item("manager")
If Not IsDBNull(rs.Item("code")) Then
code.Value = rs.Item("code")
'city.Items.FindByText(rs.Item("city")).Selected = True
End If
End If
cmd.Cancel()
sqlconn.conn.Close()

执行到usertype.Items.FindByText(rs.Item("usertype")).Selected = True就出错，提示“未将对象引用设置到对象的实例。”，这是怎么回事，要怎么解决，各位大侠帮帮忙！