Warning with voltaSpannerDuration

[Dr. med. Kai Lautenschläger]

Dear list,

when using repeats with alternatives and restricting the volta spanner to one 
or two bar I get warnings when compiling. If you uncomment the line 

\set Score.voltaSpannerDuration = #(ly:make-moment 2 1 )

in the below example, I hope you will be able to reproduce the warning. The 
same happens, when I use 

\set Score.repeatCommands = #'((volta #f))

I am using

Lilypond 2.17.4
jEdit 5.0pre1 (but it also happens, when I compile directly with Lilypond)
Mac OS 10.7.5

The PDF is showing the expected result (with correctly shortened volta 
spanners), but I still get the warning. Does anybody have any clues, why that 
is so?

Best regards

Kai