I have two workbooks. A sheet “A” create a new instance of Excel.Application, then it asks its workbook “A” to open a workbook “B” and get the result from a function in workbook “B”.
I can manage to call workbook “B”‘s function from sheet “A”, but each time I call them from workbook “A” I have an error: Object doesn’t support this property or method (Error 438)

Code that reproduce the error:

Workbook “A”

ThisWorkbook

Sub call_fun(ByRef wb As Workbook)
    MsgBox (wb.fun()) ' How to get the result?
End Sub

Sheet “A”

Sub sheet_main()
    Dim app As Excel.Application
    
    Set app = New Excel.Application
    ' Open the workbook B
    app.Workbooks.Open ("pathtofilefile workbook A.xlsm")
    ' Call the workbook's a function
    ThisWorkbook.call_fun (app.ActiveWorbook)
    
    app.ActiveWorkbook.Close
    app.Quit
    Set app = Nothing
End Sub

Workbook B

ThisWorkbook

Function fun() As String
    fun = "Function from workbook B"
End Function