programming error: cannot align on self: empty element

[Nick Payne]

The following minimal example generates the error "programming error: 
cannot align on self: empty element" in the console log.

\version "2.13.58"

\relative c' {
    \tempo 4 = 72
    \override Score.MetronomeMark #'stencil = ##f
    c4 c c c
}

Comment out either the \tempo indication or the override and the error 
goes away. I use this combination of commands in the \score block quite 
often if I want the midi output to play at a particular speed for 
proofing, but I don't want a tempo indication to appear in the score. 
Having the error appear in the log doesn't seem to affect either the 
midi output or the printed score.

Nick