programming error: minimise_least_squares (): Nothing to minimise

[nick . payne]

The full error in the console is 

programming error: minimise_least_squares ():  Nothing to minimise
This means that vertical spacing is triggered
before line breaking
continuing, cross fingers

I originally asked this question a couple of weeks ago but never got a reply. What causes this error? I have a fairly complex score where if I override a beam position with 

\once \override Beam #'positions = #'(l . r)

then this error will appear, though the output still looks fine. I've also found that as I add more bars, the error will disappear again, but then reappears later on when yet more bars are added.

I'm running 2.12.2 on Windows.

Nick Payne