รัน Query ฟ้อง System.NullReferenceException Object reference not set to an instance of an object

            For i As Integer = 0 To Me.DataGridView1.Rows.Count - 1
                Dim a As String = "ยังไม่ได้รับคืน"
                If Me.DataGridView1.Rows(i).Cells(6).Value.ToString = a Then
                    Me.DataGridView1.Rows(i).Cells(6).Style.ForeColor = Color.Red
                End If
            Next