Hiding metronome mark gives "cannot align on self: empty element"

[Nick Payne]

 If I hide the metronome mark with \override Score.MetronomeMark 
#'stencil = ##f, the console output contains:

Preprocessing graphical objects...
programming error: cannot align on self: empty element
continuing, cross fingers
programming error: cannot align on self: empty element
continuing, cross fingers

the score still builds ok. If I add a \midi { } section, or comment out 
the override, the error disappears.

\version "2.13.34"

\score {
    \relative c'' {
        \override Score.MetronomeMark #'stencil = ##f
        \tempo 4 = 72
        c c c c
    }
}

Is this a bug or expected behaviour?

Nick