Overflow Error While Selecting The Entire Worksheet

I have following code in the worksheet and every time I select the entire worksheet it generates an "Overflow" error.
Just curious to know why and if anything needs to be changed in the code to allow user to select the entire sheet.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Count <> 1 Then Exit Sub
If Target.Column = 2 Then Call detail_rep(Target.Row) ' Module 9
If Target.Address = "$A$1" Then Call Summ ' Module 1

End Sub

Thanks for any suggestions.

modytrane.