Referencing Textbox in a loop

Hi,

I wrote the below code to assign the value to a text box with the sum of every nth textbox value in the given range.

Sub GetHSum(startControl As Integer, endControl As Integer)
        Dim xsum As Integer, i As Integer, T As TextBox
        xsum = 0
        For i = startControl To endControl Step 5

            If String.IsNullOrEmpty(FindControl("Textbox" & i).text) Then
                xsum = xsum + 0
            Else
                If xsum = 0 Then
                    xsum += Convert.ToInt32(FindControl("Textbox" & i).text)

                Else
                    xsum += Convert.ToInt32(FindControl("Textbox" & i).text)
                End If
            End If
        Next
        FindControl("Textbox" & i + 5).text = xsum

    End Sub

Receiving Error for the above code.  Can some one help me on how to reference the textbox control and get the value while iterating through the loop?