ports problem after new world

After updating 5.1-CURRENT this morning I get the following error when 
trying to build any port:
Your system is too old to use this bsd.port.mk.  You need a fresh make 
world or an upgrade kit.  Please go to http://www.FreeBSD.org/ports/ or 
a mirror site and follow the instructions.

I always do my best to follow the handbook upgrade path, so I'm not sure 
if I've done something wrong or if there's a problem with a commit that 
was made within the last 24hrs.

Pete...