Runtime Error 4120 in Word 2007 macro

[Frankwlc]

I have built a protected form in Word 2007 but the eventual user wants the facility to extend the number of rows themselves in certain tables as the 'standard' amout I have set out may not be sufficient for their needs which can vary from form to form. I knew I would have to utilise a macro to do this but this is new territory for me so I did some Googling and found a suitable macro script on another forum (see script at end of this post). I tested this out on a dummy document using a keyboard shortcut and it worked fine but when I use the macro on the proper form I get a Runtime error, even though the macro does add the extra row. The proper form is not complex as it only consists of the normal protection with unprotected text fields although it is around 10 pages in length. There are no other macros that might cause a conflict.
When I check the debug info, the line oFld(i).Result = "" is highlighted as being the problem.
I have added the error and debug info below, along with the macro script.
Hoping someone can shed some light on this for me.

================================================== =========================

Sub AddARow()
Dim i As Integer
Dim bProtected As Boolean
Dim sNewRow As String
Dim oFld As FormFields
sNewRow = InputBox("Insert New Row", "New Row", "No")
If Left(UCase(sNewRow), 1) <> "N" Then
'Unprotect the file
If ActiveDocument.ProtectionType <> wdNoProtection Then
bProtected = True
ActiveDocument.Unprotect Password:=""


End If
With Selection
.SelectRow
.Copy
.Paste
.SelectRow
Set oFld = Selection.Range.FormFields
For i = 1 To oFld.Count
oFld(i).Result = ""
Next
End With
If bProtected = True Then
ActiveDocument.Protect _
Type:=wdAllowOnlyFormFields, NoReset:=True, Password:=""
End If
End If
End Sub