UnityEvent: failed to convert parameters

I’m trying to send OnTrigger data to another script using UnityEvents but this doesn’t work.
I get a failed to convert parameters error from System.Reflection.MonoCMethod.Invoke

using UnityEngine;
using UnityEngine.Events;

[System.Serializable]
public class SendTriggerMessage2D : UnityEvent<Collider2D> { }

public class TriggerMessage2D : MonoBehaviour {

    public SendTriggerMessage2D OnTriggerEnter;
    public SendTriggerMessage2D OnTriggerExit;
    public SendTriggerMessage2D OnTriggerStay;

    void OnTriggerEnter2D(Collider2D other) {
        OnTriggerEnter.Invoke(other);
    }

    void OnTriggerExit2D(Collider2D other) {
        OnTriggerExit.Invoke(other);
    }

    void OnTriggerStay2D(Collider2D other) {
        OnTriggerStay.Invoke(other);
    }
}

Never figured it out. Nor do I remember what I was using this for. Heh.