Been trying to make the "Expire date" property (stoppublish) required for a specific page type (or all of them for that matter)

Found this nice blog post from Kalle and ended up with code looking like this:

    public class SiteMetadataExtender : IMetadataExtender 
    { 
        public void ModifyMetadata(ExtendedMetadata metadata, IEnumerable attributes) 
        { 
            foreach (ExtendedMetadata property in metadata.Properties) 
            {
                if ((property.PropertyName == "TeaserText" || property.PropertyName == "iversionable_stoppublish") && property.Parent.Model is CalendarPage) 
                { 
                    property.IsRequired = true;
                } 
            } 
        } 
    }

This works for the "TeaserText" property that I have defined myself on the CalendarPage but the built-in "iversionable_stoppublish" gets ignored. 

So is there some magic that I'm missing when working with the built-in properties?