Unity accepts default parameter in event subscription but fails

I encountered a really strange problem with events. I seems that Unity's compiler has a problem when it comes to default parameters.

I have an Event:

 public static event System.Action<int> OnVisualLevelUp;

and I have a Function:

 public static void LevelAchievement(int playerLevel, System.Action potentialAbordAction = null, bool autoLogin = false)

Then I do this:

 LevelBar.OnVisualLevelUp += WK_GameCenter.LevelAchievement;

MonoDevelop complaints about that, but Unity does not, so I left it as is.

When the event is called, LevelAchievement does this and throws a NullReferenceException on the last line:

    if (potentialAbordAction == null){
       Debug.Log("potentialAbordAction is null");
       return;
    }
    potentialAbordAction ();

It actually reaches the function call, not returning on the check. I fixed it by adding the missing parameters to the event, but how is this even possible?

I would be very satisfied if somebody could explain that to me :)