Check if Excel is open in Word VBA

[RosemaryBaby]

Hi everyone,
I am stucked with the issue like the title. Any one help me?
it always pop an error "subscript out of range"
Here is my code in Word VBA:

Sub OpenExcel()
'Checking if Appendix data source Excel file is already in open state
If IsExcelOpen("a.xls") Then
Workbooks("a.xls").Close 'The issue is here
Else
'Opening Excel workbook with Appendix Tables
Set wbWorkBook = appExcel.Workbooks.Open("a.xls")
End If
End Sub

'Checking if Excel file is open
Function IsExcelOpen(strFileName As String) As Boolean
Dim lFilenum As Long
Dim lErr As Long

On Error Resume Next
lFilenum = FreeFile()
Open strFileName For Input Lock Read As #iFilenum
Close iFilenum
lErr = Err
On Error GoTo 0

Select Case lErr
Case 0: IsExcelOpen = False
Case 70: IsExcelOpen = True
Case Else: Error lErr
End Select