AI::Categorizer restore problem

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Hi folks,  
 
I'm trying to implement AI:Categorizer. Everything works fine 
until I try to save and restore the state of the learner. 
 
>From the synopsis... 
 
 my $nb = new 
AI::Categorizer::Learner::NaiveBayes(...parameters...); 
 $nb->train(knowledge_set => $k); 
 $nb->save_state ('filename'); 
 
--- later ---- 
 
 $nb = 
AI::Categorizer::Learner::NaiveBayes->restore_state('filename'); 
 my $c = new AI::Categorizer::Collection::Files( path => ... ); 
 while (my $document = $c->next) { 
   my $hypothesis = $nb->categorize($document); 
   print "Best assigned category: ", 
$hypothesis->best_category, "\n"; 
   print "All assigned categories: ", join(', ', 
$hypothesis->categories), "\n"; 
 
However when the line  
my $hypothesis = $nb->categorize($document); 
is executed I get the following error. 
 
Can't locate object method "predict" via package 
"Algorithm::NaiveBayes::Model::Frequency" 
at /usr/lib/perl5/site_perl/5.8.3/AI/Categorizer/Learner/NaiveBayes.pm 
line 28. 
 
It seems like inheritance is lost between save and restore 
states. Any ideas? 
 
Thanks in advance.  
 
PS: CC to email highly appreciated.