Writing a value to text file is being wrongly displayed

I have the following block of simple code which writes the row index of the current selected cell. The odd thing is, when I perform the code, sometimes it will set the correct index in the textfile e.g. row 1 is index 0 I believe, yet other times I will perform it on the same row 1 and the index will be set to 640 or another incorrect number.

Do you see anything wrong:

'This is the code which writes to the text file:
        If Main.TabControl1.SelectedIndex = "3" Then My.Computer.FileSystem.WriteAllText("C:\Old Time Radio\MyFavoriteOTR\Texts\CoupleNextDoor.txt", _
Main.CoupleNextDoorDataGridView.CurrentRow.Index, True)
 
'This is the code which reads from the file (this works fine but is added for clarity)
 
fileReader = My.Computer.FileSystem.ReadAllText("C:\Old Time Radio\MyFavoriteOTR\Texts\CoupleNextDoor.txt")
CoupleNextDoorDataGridView.FirstDisplayedScrollingRowIndex = fileReader

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