I've never seen this behavior before.  Like ever.  So I'm confused. 

You can look at the source, as you're doing.  You may also want to check the way your do your NURBS setup.  It occurs to me that if something is happening in that call, it may be because it's not be getting data in an anticipated way.  In all honesty, I haven't done NURBS rendering in years, so I'm sorry, but I can't be much help there.