Help please!

[Trevor Daniels]

I'm drafting out a new engraver to produce durations above a tab 
staff for use in lute tablature, and I've been stumped by a problem 
for several hours.  Perhaps someone can see something I'm missing.

The problem is this (recast in a simplified form):

If I set the text property in a new grob like this,
the A is engraved correctly:

 Item *duration_grob;
 ...
 duration_grob = make_item ("TabDuration", events_[0]->self_scm 
());
 ...
 duration_grob->set_property ("text", ly_string2scm ("A"));

but if I do this:

 SCM duration_symbol;
 Item *duration_grob;
 ...
 duration_grob = make_item ("TabDuration", events_[0]->self_scm 
());
 ...
 duration_symbol = ly_string2scm ("A");
 duration_grob->set_property ("text", duration_symbol);

the program seg faults way down in the bowels.
The last few frames are

#0  0x00cf610d in scm_gc_mark_dependencies () from 
/usr/lib/libguile.so.17
#1  0x00cf64ac in scm_gc_mark () from /usr/lib/libguile.so.17
#2  0x00cf63d8 in scm_gc_mark_dependencies () from 
/usr/lib/libguile.so.17
#3  0x00cf64ac in scm_gc_mark () from /usr/lib/libguile.so.17

Why?

Trevor