tooltip event returns zero length

I am getting a tooltip error if Length cannot be less than zero.  On some fields I hav not text others have anywhere from 4 to 2000 characters.  how can i get by this error.
 
 Private Sub myDataGrid_ItemDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.DataGridItemEventArgs) Handles myDataGrid.ItemDataBound
        If e.Item.ItemType = ListItemType.Item OrElse e.Item.ItemType = ListItemType.AlternatingItem Then
            Dim cellType As TableCell = e.Item.Cells(7)
            cellType.ToolTip = cellType.Text
            If cellType.Text.Length > 1 Then cellType.Text = cellType.Text.Substring(100) & "..."
        End If
    End ...