What cause Error 4120?

Combining some code discussed in my previous Two Questions post, I have
put together a macro to validate and tally columns of checkboxes in a
table.  To duplicate, create a 4 column, 5 row table.  Leave the first
and last rows blank and the first column blank.  Put checkboxes in the
remaining cells:

The following macro is set to run from a macrobutton in the document:

Sub ValidateAndTally()
Dim pTable As Word.Table
Dim pFormField As FormField
Dim pColumnCount&, pColumn&, i&, oLastRow
Dim pCount() As Long
Dim pColumnFlags() As Boolean
Dim x As Long, y As Long, z As Long
Dim bValidator As Boolean
Set pTable = ActiveDocument.Tables(1)
pColumnCount = pTable.Columns.Count
oLastRow = pTable.Rows.Count
ReDim pCount(1 To pColumnCount)
ReDim pColumnFlags(1 To pColumnCount)
On Error GoTo Handler
ActiveDocument.Unprotect
For Each pFormField In pTable.Range.FormFields
 pColumn = pFormField.Range.Cells(1).ColumnIndex
 If pFormField.Type = wdFieldFormCheckBox Then
   pColumnFlags(pColumn) = True
   If pFormField.CheckBox.Value Then
     pCount(pColumn) = pCount(pColumn) + 1
   End If
 End If
Next
bValidator = True
For x = 2 To pTable.Rows.Count - 1
 With pTable.Rows(x)
   z = 0
   For y = 2 To pTable.Columns.Count
     If .Cells(y).Range.FormFields(1).CheckBox.Value = True Then
       z = z + 1
     End If
   Next y
   Select Case z
     Case Is = 0
       MsgBox "You did not answer question " & x - 1 & "."
       bValidator = False
       Exit For
     Case Is > 1
       MsgBox "You checked more than one answer to question " & x - 1
& "."
       bValidator = False
       Exit For
   End Select
 End With
Next x
For i = 1 To pColumnCount
 If pColumnFlags(i) Then
   If bValidator Then
     pTable.Cell(oLastRow, i).Range.Text = pCount(i)
   Else
     pTable.Cell(oLastRow, i).Range.Text = "Not Validated"
   End If
 End If
Next
ActiveDocument.Protect wdAllowOnlyFormFields, NoReset:=True
Exit Sub
Handler:
If Err.Number = 4605 Then
 ActiveDocument.Protect wdAllowOnlyFormFields, NoReset:=True
 Resume
Else
 MsgBox Err.Number & " " & Err.Description
End If
End Sub

When I get it working, it seems to work as expected.  However, when I
unprotect and edit the form then reprotect, I almost always get errror
4120 bad parameter when I try to fire the macro with the macrobutton.
If I then go into the editor and run the macro it runs fine and
thereafter runs from the macrobutton without further problem.

What is this bad parameter and how do I prevent it from happening.
Thanks.