Whole note tremolo repeats - bug ?

[Lasse Rempe]

The following lilypond code:

c''''1:16

should (in my opinion) produce a whole note, with the slashes for 
tremolo repeats inside the staff (see the attached scan). However, 
lilypond creates something which has the tremolo indication in the 
ledger lines (see second image). This doesn't look good - is it a bug, 
or am I missing something?

Thanks,
Lasse