no tuplet bracket, why?

[Stefan Thomas]

Dear Lilypond-users,
is there any reason, why in the below quoted example the tuplet bracket is hidden?
I couldn't find a way to make it visible!
{\clef bass r2
 <a, a> 4 r r  \times 2/3 { r8 <a, a>4 } r4 r \times 2/3 { r4 <a, a>8 } r4 r r }