vba error subscript out of range

Hello Everyone

Dim LastRec As Long

Sheets("Data").Select
LastRec = ThisWorkbook.Sheets("Data").UsedRange.Rows.Count

    
    For i = 1 To LastRec
            Cells(i, 3).Select
            StrTxt = Selection.Text
            ActiveCell.Offset(0, 4).Select
            ExTxt = Selection.Text
                Do Until Cells(i + 1, 4).Value < Cells(i, 4).Value
                StrTxt = StrTxt & " " & ExTxt
                i = i + 1
                Loop
    Next i

I get error on following line;

LastRec = ThisWorkbook.Sheets("Data").UsedRange.Rows.Count

Kindly help