Word 2010 Macro to Merge Files Not Working

This macro is to merge all the files in a folder into the current document BUT I would also like to be able to tell it a different folder OR even better to tell it to merge only selected files in a folder.  Number one goal is to get the merge working.

Sub MergeDocs()
Dim rng As Range
Dim MainDoc As Document
Dim strFile As String
Const strFolder = "U:\MergeDocs\"
Set MainDoc = Documents.Add
strFile = Dir$(strFolder&"*.docx")
Do Until strFile = ""
Set rng = MainDoc.Range
rng.Collapse wdCollapseEnd
rng.InsertFile strFolder & strFile
strFile = Dir$()
Loop
End Sub

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Word-Macro-Merge-File.jpg