Errors when using TabStaff with stringTunings

[Kevin Nowaczyk]

I have a lilypond file which used to parse perfectly well several years ago. I 
recently reinstalled lilypond (2.18.2 from debian-stable) and now I receive an 
error. I've reduced the file down and determined that setting the stringTunings 
is the problem. The file I have has both guitar tab and bass guitar tab. If I 
leave the   \set TabStaff.stringTunings = #'(4 -1 -5 -10 -15 -20) line out, it 
displays correctly. If I add it in, it presents the following error:

GNU LilyPond 2.18.2
Processing `Steven_smith_working.ly'
Parsing...
Interpreting 
music...[8]/usr/share/lilypond/2.18.2/scm/translation-functions.scm:313:39: In 
procedure ly:pitch-tones in expression (ly:pitch-tones (list-ref tuning #)):
/usr/share/lilypond/2.18.2/scm/translation-functions.scm:313:39: Wrong type 
argument in position 1 (expecting Pitch): 4


My test file:

guitarmusic =  {
        \key g \minor
        R1*12
        r8 bes'8\1 bes'\1 bes'\1 r8 a'\1 a'\1 a'\1 r c''\1 c''\1 c''\1 r bes'\1 
bes'\1 bes'\1
        r bes'\1 bes'\1 bes'\1  r a'\1 a'\1 a'\1 r d''\1 d''\1 d''\1 r d''\1 
d''\1 d''\1
        r8 bes'8\1 bes'\1 bes'\1 r8 a'\1 a'\1 a'\1 r c''\1 c''\1 c''\1 r bes'\1 
bes'\1 bes'\1
        r bes'\1 bes'\1 bes'\1 r a'\1 a'\1 a'\1 d''\1 d''\1 d''\1 d''\1 d''\1 
d''\1 d''\1 d''\1
}

\version "2.12.3"
\score {
        \new StaffGroup <<
                \new TabStaff <<
                        \set TabStaff.stringTunings = #'(4 -1 -5 -10 -15 -20)
                        {
                                \guitarmusic
                        }
                >>
        >>
        \layout{}
}

Obviously, if all I'm doing is standard-tuned guitar, I can leave out the call 
to stringTunings, but for bass guitar I need that call.
Thanks for any help,
Kevin Nowaczyk