AddOLEObject does not accept my filename variant

I have the following VBA subroutine in a word document:

Private Sub InsertPDF(fullFileName, displayName)
    Selection.InlineShapes.AddOLEObject ClassType:="AcroExch.Document.DC", _
        FileName:=fullFileName, _
        LinkToFile:=False, _
        DisplayAsIcon:=True, _
        IconFileName:="C:\Windows\Installer\{AC76BA86-7AD7-1033-7B44-AC0F074E4100}\PDFFile_8.ico", _
        IconIndex:=0, _
        IconLabel:=displayName
End Sub

When I run it, it errors with the message: "word cannot obtain the data for the <file> link".

While debugging the locals window show the variable with the correct path, which is evidenced by copying it directly from there and hard-coding fullFileName in the routine to the value set, e.g. "C:\users\user\file.pdf" the code works. The Type shows as "Variant/String", which I believe it correct. The variable displayName is the same type and it does not need to be hard-coded.

I have tried to sanity check the string by running it through Trim and CStr without success.

What am I doing wrong, or what else can I do to narrow down the problem?

There was a trailing CRLF at the end of the string. I normalised the string on input like so; note the additional line at the top of the sub:

Private Sub InsertPDF(fullFileName, displayName)
    fullFileName = Replace(fullFileName, vbCrLf, "")
    Selection.InlineShapes.AddOLEObject ClassType:="AcroExch.Document.DC", _
        FileName:=fullFileName, _
        LinkToFile:=False, _
        DisplayAsIcon:=True, _
        IconFileName:="C:\Windows\Installer\{AC76BA86-7AD7-1033-7B44-AC0F074E4100}\PDFFile_8.ico", _
        IconIndex:=0, _
        IconLabel:=displayName
End Sub