Error 4605 "this method or property is not available because there is a memory or disk error" upon running Macro

This is the MS-Word Macro. It produces this error:

Error 4605 "this method or property is not available because there is a memory or disk problem

' Word script to remove all unused styles in the document
Sub DeleteUnusedStyles()
Dim Doc As Document, bDel As Boolean
Dim Rng As Range, StlNm As String, i As Long
Application.ScreenUpdating = False
Set Doc = ActiveDocument
With Doc
  For i = .Styles.Count To 1 Step -1
    With .Styles(i)
      If .BuiltIn = False Then
        bDel = True: StlNm = .NameLocal
        For Each Rng In Doc.StoryRanges
          With Rng
            With .Find
              .ClearFormatting
              .Format = True
              .Style = StlNm
              .Execute
            End With
            If .Find.Found = True Then
              bDel = False
              Exit For
            End If
          End With
        Next
        If bDel = True Then .Delete
      End If
    End With
  Next
End With
Application.ScreenUpdating = True
End Sub

It should remove all unused styles from a word document. I have found on the internet and tested 3 other macros that ought to do the same thing, but with the same error coming up in the end. The Word document pauses for about 5 minutes (it's 100 pages document heavily formatted) and then it spits out this error. I can see memory consumption for the word process increasing almost 5 times before the error appears. I still have more than enough RAM though. I run authentic Word 2013 on Windows 8.1 x86_64. Why is this happening and how do i fix the error?

In this case it is obviously the Undo stack clogging up.

Simple fix: Amend the code like this:

        '...
        '...
        Next
        If bDel = True Then .Delete
        Doc.UndoClear
    End If
End With