warning with percent repeats

[Graham Percival]

The following file

{
  c''2
  \repeat percent 2 { c''8 d'' }
}

compiles with a warning:
-----
GNU LilyPond 2.4.2
Processing `foo.ly'
Parsing...
Interpreting music... warning: Don't know how to handle a percent 
repeat of this length.
[2]
Preprocessing graphical objects...
Calculating line breaks... [2]
Layout output to `foo.tex'...
Converting to `foo.dvi'...
Converting to `foo.ps'...
Converting to `foo.pdf'...
-----

The output looks ok.

Does this warning matter?  Does that example have wrong syntax?  This 
is going
in the manual, so it would be nice to have good syntax.  :)

Cheers,
- Graham