Run-time error '9': Subscript out of range

I am trying to get the following code to work:
<<
Sub CallSetPropertyFunction()

Dim PropertyName As String
Dim PropertySet As PropertyLocation
Dim PropertyValue As Variant
Dim ContentLink As Boolean
Dim TargetWorkBook As Workbook

PropertyName = "Medical"
PropertySet = PropertyLocationBoth
PropertyValue = "Hospital"
ContentLink = False

Workbooks.Open ("N:\!DocumentDayFile\1SetPropertyTest.xlsx")

Set TargetWorkBook = Workbooks("N:\!DocumentDayFile\1SetPropertyTest.xlsx")
<<

The line starting "Workbooks.Open..." works in that it opens the correct file thus proving that the file exists.

The line starting "Set TargetWorkBook..." results in the above run time error.

I am new to VBA programming but studies so far indicate that this error occurs when the target does not exist but the previous line opens the file successfully thus proving that it does.

The ultimate project objective is to create custom file properties to scanned pdf files and office documents and set their values. This is just the first step.
Any help would be appreciated.

Many thanks CK76 for your prompt reply and please accept my apologies for the dilatory response; I have been unwell.

I thought I had already tried your suggestion but apparently not, either that or there was a typo in my original attempt. I did not realise however that I could do it in one line; most useful.

This leads me to believe that the value of a workbook object can only be an open file name; is that correct?