Question mark when rendering half-diminished 7th chord

[Nils Olsson]

Following the notation reference I've notated a C-sharp half-diminished 
seventh chord in chord mode as such:

\chords { cis4:m7.5- }

But when I compile a PDF the result is the letter (C#) with a question 
mark instead of the slashed-o (Ø). I've tried compiling with different 
fonts and it doesn't seem to be a PDF viewer error. Any ideas?

Thanks,
Nils