This popped up last night when, IIRC, I set my large castle to be built. Didn’t seem to cause/break anything, but there we go…
can confirm… I wasn’t building something I’d classify as a “castle”, more a hovel… 
but still, had the same result when I initiated the construction…
I’ve started building a mega build as well to check what happens to my framerate. As soon as I’ve started the building, I’ve got such an error, too.
