puzzling JSON bug

David Wheeler has presented me with a nasty bug case.

If I do this:
   select '{"members": { "add": [3, 4]}}'::json #> '{members,add}';

then I get a crash.

If I comment out the pfree() at json.c:parse_object_field() lines 378-9 
then I get back the right result but instead get a warning like this:
   WARNING:  problem in alloc set ExprContext: bogus aset link in block   0x1efaa80, chunk 0x1efb1f0


I'm not quite sure where I should go looking for what I've done wrong here.

cheers

andrew