Closing a Workbook from another workbook: Workbooks.close error: Subscript out of range

Hello,
In the code below, I first check to see if a certain workbook is open from a separate workbook. If it is, I would like to save and close it.
The workbook is open, but when it attempts to save and close it (in line Workbooks("S:\Public\PMG\Fixed Income\FI-AA\Attribution\AttributionUS2013v3.xlsm").Close), I recieve Run Time Error '9': Subscript out of range.

Any ideas on how I can solve this?
Thank you in advance

Sub Sample()
Dim Ret

Ret = IsWorkBookOpen("S:\Public\PMG\Fixed Income\FI-AA\Attribution\AttributionUS2013v3.xlsm")

If Ret = True Then
Workbooks("S:\Public\PMG\Fixed Income\FI-AA\Attribution\AttributionUS2013v3.xlsm").Close True
End If
End Sub

Function IsWorkBookOpen(FileName As String)
Dim ff As Long, ErrNo As Long

On Error Resume Next
ff = FreeFile()
Open FileName For Input Lock Read As #ff
Close ff
ErrNo = err
On Error GoTo 0

Select Case ErrNo
Case 0: IsWorkBookOpen = False
Case 70: IsWorkBookOpen = True
'Case 75: IsWorkBookOpen = True 'this is read only open
Case Else: Error ErrNo
End Select
End Function

Unfortunately, I get the same error whether I use:
Workbooks("S:\Public\PMG\Fixed Income\FI-AA\Attribution\AttributionUS2013v3.xlsm").Close True or
or
Workbooks("AttributionUS2013v3.xlsm").Close True

Any ideas on why that might be?
Thanks.

Tracing through the code in debug mode, the file is indeed open- the if statement triggers.
The code throws an error on the Workbooks("AttributionUS2013v3.xlsm").Close True line.
I agree with you that this is a very odd error, I searched Google for quite awhile and couldnt find a similar problem.

Haha very true- it is just odd to me.
The workbook is open in a separate instance of excel. So once again, I am not sure what is causing this issue. Seems like it should be straight forward, but there is obviously something I am missing .

Not sure if it matters, but the workbook which is checking to see if the other workbook is open, then saving/closing it, is on my desktop, while the workbook to be saved/closed is on a shared drive. I don't believe that should make a difference, but this problem is very baffling.
Both are open in separate instances of excel.
I appreciate your help, hopefully this can be solved.

Awesome that did it. Thank you so much! And apologies, I realized I misread your previous statement about different instances.
Cheers