problem with last line of a table  in Microsoft Word

Occasionally, the bottom gridline of a table in Microsoft Word disappears (see image below). I have tried many ways to correct this: reformat the table, add a line, etc but nothing works to get the gridline back.

When I use the following code:

Sub GetAvg()
Dim cellLoop As Cell, CellContents, datePreviousVisit, intColumnIndex As Integer

intColumnIndex = Selection.Cells(1).ColumnIndex
Selection.GoTo wdGoToBookmark, , , "VitalSigns"
Selection.MoveDown wdLine, 1
Selection.HomeKey wdLine
Selection.MoveRight wdWord, 5, wdExtend
If IsDate(Selection.Range.Text) Then
    datePreviousVisit = InputBox("Enter Fill date:", , CDate(Selection.Range.Text))
Else
    datePreviousVisit = InputBox("Enter Fill date:")
End If
If Len(datePreviousVisit & "") = 0 Then Exit Sub Else datePreviousVisit = CDate(datePreviousVisit)
'Debug.Print Format(datePreviousVisit, "mmm dd, yyyy")
For Each cellLoop In ActiveDocument.Tables(3).Columns(intColumnIndex).Cells
        CellContents = Split(Left(cellLoop.Range.Text, Len(cellLoop.Range.Text) - 2), ",")
        If UBound(CellContents) = 1 Then
            cellLoop.Range.Text = Trim(CellContents(0))
            If CellContents(1) = "" Then CellContents(1) = 0
            cellLoop.Range.Text = Trim(CellContents(0)) & ", " & Trim(CellContents(1)) & ", " & Trim(Round((cellLoop.Range.Calculate - CellContents(1)) / DateDiff("d", datePreviousVisit, ActiveDocument.Bookmarks("DOS").Range.Text), 1))
        End If
Next cellLoop
ActiveDocument.Tables(3).Cell(1, intColumnIndex).Select
End Sub

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with the missing gridline, I get this error message: "run time error 5992; Cannot access individual columns  in this collection because the table has mixed cell widths."

Is there a way to get the gridline back or a way to change the code so it runs with the missing gridline?

Thanks