No binary compatibility, although the declaration is kept the same

I am trying to create a new version of COM DLL with binary compatibility. I don't understand why I am getting this message:

'init' in the 'Logger' class module has arguments and/or a return type that is incompatible with a similar declaration in the version-compatible component.

Original definition:
 Function init(aLOGDIR As String, Optional aListBox As Object, Optional aMAXLISTBOXLINES As Integer) As Boolean

Current definition:
 Function init(aLOGDIR As String, Optional aListBox As Object, Optional aMAXLISTBOXLINES As Integer) As Boolean

      

        
        
        
      

    

I am not changing init

, as you can see ...

Here's my steps:

• First generation without compatibility
• Setting up binary compatibility in the project properties (link to the previously generated dll with or without renaming it)
• Second generation
• Warning.

Is it because the parameter Object

? Thanks for your help.