program error: mis-predicted force

[Partitura Organum]

Hi all,

Every now and again I see this error when engraving a score"program 
error: mis-predicted force" and then a bunch of numbers. But the score 
looks okay so I ignore it. And since the error never comes with a line 
in the music file where the error originates I never know what is 
actually wrong.

Tonight it happened again. This time I have been able to narrow down the 
source of the error to the next snippet (sort of "Minimal Error Throwing 
Example") :

%%%%%% start

\version "2.19.59"

#(set-global-staff-size 18)

\score {
  \new Staff
    \relative c'' {
      \key bes \major
       es4 d c8 cis d bes
      \grace {a16 c}
  }
}

%%%%%%%% end

All elements are needed for the error to occur. With another 
global-staff-size: no error. Remove one of the notes: no error. Remove 
the key signature: no error. Remove a grace note: no error

So, what am I doing wrong here?

Regards,
Auke