Didn't find a vertical alignment in this system

[Nick Payne]

I have a score on which I'm using showLastLength to only generate the 
last ten bars. As I add bars and run Lilypond, every so often I get the 
error below and the score won't build until I change the number of bars 
for showLastLength. What is the error indicating? I had a quick look at 
page.scm but I can't see why page layout should suddenly barf due to 
starting output at some point into the score. LP 2.13.22 x86 on Linux:

"programming error: didn't find a vertical alignment in this system
programming error: system with empty 
extent/usr/local/lilypond/usr/share/lilypond/current/scm/page.scm:244:68: In 
procedure - in expression (- 0 y ...):"