error message I don't understand

[Stefan Thomas]

Dear lilpond-users,
when compiling the below quoted file I get the error message:
warning: type check for `stencil' failed; value `#t' must be of type `unknown'
I think it has to do with the tuplets, but I don't understand, what
I've done wrong. Here is the snippet:


\version "2.11.65."
klammernormal = {  \override TupletBracket #'bracket-visibility = #'if-no-beam }
% Klammer und Zahl werden  ganz normal gezeigt
klammeran =  { \override TupletBracket #'bracket-visibility = ##t }
klammeraus = { \override TupletBracket #'bracket-visibility = ##f }
mitzahl ={ \override TupletNumber #'stencil = ##t }
ohnezahl = { \override TupletNumber #'stencil = ##f }
standardxtol = { \klammernormal \mitzahl }

\relative {
\times 2/3  { c8  d e } \ohnezahl \times 2/3 { f e f } \standardxtol
\times 2/3  { d e f }
\times 2/3 { g f g } }