Why can't I get this to work (viola tuning)

[Julian Vincent]

\version "2.10.0"
\new TabStaff <<
  \set TabStaff.stringTunings = #'( 9 2 -5 -12)
  { c g d' a'}
>>

This is the open tuning for a viola.
I've tried all sorts of combinations and I can't get the bottom C to  
register as an open string, though if I put in an extra string, even  
with the same value (-12) then I get the bass string registering as  
open.  What am I doing wrong??
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