error with mongo on ruby

I am trying to run a map-reduce and it is failing with the following
error :

/usr/lib/ruby/gems/1.8/gems/mongo-1.4.0/lib/../lib/mongo/db.rb:520:in
`command': Database command 'mapreduce' failed: (errmsg: 'assertion:
assertion scripting/engine_spidermonkey.cpp:433'; ok: '0.0').
(Mongo::OperationFailure)
  from /usr/lib/ruby/gems/1.8/gems/mongo-1.4.0/lib/../lib/mongo/
collection.rb:618:in `map_reduce'
  from unique-sites.rb:164

The map reduce works okay if a smaller dataset is provided. Can
someone please help ?

Thanks
Pankaj

HEre are the map and reduce functions:

map = "function() { if ( this.sites ) " +
                 "{ " +
                   " var parent_deviceid=this.deviceid; "+
                   " var unique_total_dur = new Object(); " +
                   " this.sites.forEach(function(site_entry) { " +
                   " var validDur = 0; "+
                   " if ( site_entry.duration ) "+
                   " validDur = site_entry.duration; "+
                   " if ( site_entry.url in unique_total_dur ) " +
                        " unique_total_dur[site_entry.url] +=
validDur; "+
                   " else { " +
                          " unique_total_dur[site_entry.url] = 0; "+
                          " unique_total_dur[site_entry.url] +=
validDur; "+
                      "}}); " +
                     " for( var visited_site in unique_total_dur ) { "