Why are COM event handlers always null?

Using this article, I've set up this COM-visible interface to define my events:

[ComVisible(true)]
[Guid("3D8EAA28-8983-44D5-83AF-2EEC4C363079")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IParserStateEvents
{
    void OnParsed();
    void OnReady();
    void OnError();
}

The events are meant to be fired by a class that implements this interface:

[ComVisible(true)]
public interface IParserState
{
    void Initialize(VBE vbe);

    void Parse();
    void BeginParse();

    Declaration[] AllDeclarations { get; }
    Declaration[] UserDeclarations { get; }
}

Here's the implementation:

[ComVisible(true)]
[Guid(ClassId)]
[ProgId(ProgId)]
[ClassInterface(ClassInterfaceType.AutoDual)]
[ComDefaultInterface(typeof(IParserState))]
[ComSourceInterfaces(typeof(IParserStateEvents))]
[EditorBrowsable(EditorBrowsableState.Always)]
public class ParserState : IParserState
{
    //...
    public event Action OnParsed;
    public event Action OnReady;
    public event Action OnError;

    private void _state_StateChanged(object sender, System.EventArgs e)
    {
        var errorHandler = OnError; // always null
        if (_state.Status == Parsing.VBA.ParserState.Error && errorHandler != null)
        {
            errorHandler.Invoke();
        }

        var parsedHandler = OnParsed; // always null
        if (_state.Status == Parsing.VBA.ParserState.Parsed && parsedHandler != null)
        {
            parsedHandler.Invoke();
        }

        var readyHandler = OnReady; // always null
        if (_state.Status == Parsing.VBA.ParserState.Ready && readyHandler != null)
        {
            readyHandler.Invoke();
        }
    }
    //...

The _state_StateChanged handler is responding to events raised from a background worker thread.

The COM client code is a VBA class looking like this:

Private WithEvents state As Rubberduck.ParserState

Public Sub Initialize()
    Set state = New Rubberduck.ParserState
    state.Initialize Application.vbe
    state.BeginParse
End Sub

Private Sub state_OnError()
    Debug.Print "error"
End Sub

Private Sub state_OnParsed()
    Debug.Print "parsed"
End Sub

Private Sub state_OnReady()
    Debug.Print "ready"
End Sub

While everything looks right from the Object Browser:

...when the VBA code calls BeginParse, breakpoints get hit in the C# code, but all handlers are null, and so the VBA handlers don't run:

What am I doing wrong?