初学NET出错

 conn.Open()
            sql = "select * from WIPManagementInOut where OutTime is null"
            Dim da As OleDbDataAdapter = New OleDbDataAdapter(sql, conn)
            da.Fill(ds, "WIPManagementInOut")
            DataGridView1.DataSource = ds.Tables(0)
            DataGridView1.Refresh()
            For i As Integer = 0 To Me.DataGridView1.RowCount - 1
                If DateDiff(DateInterval.Minute, Me.DataGridView1.Item(6, i).Value, Now()) > 3 Then ‘为什么在这里出错？求指点
                    Me.DataGridView1.Rows(i).DefaultCellStyle.BackColor = Color.YellowGreen
                End If
            Next
’主要是Me.DataGridView1.Item(6, i).Value 这里出错，不知道为什么