求助：repeater控件中使用checkbox的问题

我用repeater控件绑定了一组数据，里面包括checkbox控件,当我把其中的一条记录选后，点更新按钮时，提示错误信息：
“未将对像设置引用到实例”

    Private Sub btnupdate _Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnupdate .Click
        Dim c As Control
        For Each c In Me.Repeater1.Controls
            Dim check As CheckBox = CType(c.FindControl("CheckBox1"), CheckBox)
If check.Checked = True Then
                Dim strsql As String = "update maillist set MFlag='d'"
                Dim cmd As New SqlCommand(strsql, conn)
                conn.Open()
                cmd.ExecuteNonQuery()
                conn.Close()
            End If
        Next c
    End Sub

请问该如何解决啊?            先谢谢各位了~~~~