The implementation of EventTrigger's RegisterEvent method, as follows
[code]

    private void RegisterEvent(object source, string eventName)
    {
        if (source != null && !string.IsNullOrEmpty(eventName))
        {
            Type type = source.GetType();
            EventInfo ev = type.GetEvent(eventName);

            if (ev == null)
            {
                if (SourceObject != null)
                {
                    throw new ArgumentException(string.Format(
                        "EventTrigger cannot find event '{0}' in SourceObject '{1}'",
                        eventName, type));
                }
            }

            if (!IsValidEvent(ev))
            {
                if (SourceObject != null)
                {
                    throw new ArgumentException(string.Format(
                        "SourceObject event '{0}' is not valid for EventTrigger",
                        eventName));
                }
            }
            else
            {
                _event = ev;
                _handler = Delegate.CreateDelegate(ev.EventHandlerType, this, OnEventMethod);
                _event.AddEventHandler(source, _handler);
            }
        }
    }

[/code]
doesn't properly handle the case where the default target object (usually AssociatedObject) exists but the designated event does not while no SourceObject is set (and thus SourceObject == null). As described above, this code will fall through with a null ev and null SourceObject and pass IsValidEvent null, which fails with a nullreferenceexception. An additional branch should be added to the condition to provide an informative message in this case.