crash on repeated search

I have found another crash in ferret; this one just uses a regular 
search. It's similar to an issue reported by Matt Schnitz a while ago, 
but unlike his, mine does not go away if I turn off omit_norms. It does 
go away if I turn on the garbage collector more often, but I'm not sure 
that's a stable workaround under the circumstances.

This one isn't a segfault, but more complicated versions have been. This 
bug manifests as a error in the garbage collector:

$ ruby rw_ferret_test.rb
.......rw_ferret_test.rb:20: [BUG] rb_gc_mark(): unknown data type 
0x28(0x402ba3a4) non object
ruby 1.8.2 (2005-04-11) [i386-linux]

Aborted


Here's the code:

require 'rubygems'
require 'ferret'
include Ferret

def phrase(num)
   "a_fairly_long_phrase%05d"%num
end

LOOPS=(ARGV[1]||40).to_i

_fields = Index::FieldInfos.new
_fields.add_field :phrase, :term_vector => :no, :store => :no, :index => 
:untokenized
$phrases=I.new(:field_infos=>_fields)

phrase(0).upto(phrase(LOOPS)){|phrase| $phrases << {:phrase=> phrase} }

@phrases=Ferret::Search::Searcher.new $phrases.reader
count=0
loop{
     @phrases.search(
      Search::TermQuery.new(:phrase, 
"a_fairly_long_phrase%05d"%rand(LOOPS+1))
     )

     if ((count+=1)%1000).zero? #1309
       print '.'
       STDOUT.flush
#      GC.start   #enable this to workaround
     end
   }
$phrases.close