map/reduce function leading to an assertion failure in mongodb 1.6.x

    m = Code((u'function() {\n'
             u'   var langs = {};\n'
             u'   langs[this.language] = 1;\n'
             u'   var authors = {};\n'
             u'   authors[this.author] = 1;\n'
             u'   var count_followers = 0;\n'
             u'   var count_mentions = {};\n'
             u'   if ("twitter" in this.extra_info) {\n'    # Twitter-specific things
             u'      if (this.extra_info.twitter.count_followers == null) {\n'
             u'         count_followers = 180;\n'           # this is the average number of Twitter followers
             u'      } else {\n'
             u'         count_followers = this.extra_info.twitter.count_followers;\n'
             u'      }\n'
             u'      if ("mentions" in this.extra_info.twitter) {\n'
             u'         for (var i in this.extra_info.twitter.mentions) {\n'
             u'            var screen_name = this.extra_info.twitter.mentions[i];\n'
             u'            count_mentions[screen_name] = 1;\n'
             u'         }\n'
             u'      }\n'
             u'   }\n'
             u'   var search_terms = [%s];\n'               # Count search terms separately
             u'   if (this.st_matches) {\n'
             u'      var match_st = false;\n'
             u'      for (var st in search_terms) {\n'
             u'         for (var st_match in this.st_matches) {\n'
             u'            if (st == st_match) {\n'
             u'               match_st = true;\n'
             u'               break;\n'
             u'            }\n'
             u'         }\n'
             u'      }\n'
             u'      if (match_st) {\n'
             u'         emit(null,\n'
             u'            {"count_blog_inlinks": this.blog_inlinks_count,\n'
             u'             "count_microblog_inlinks": this.microblog_inlinks_count,\n'
             u'             "count_postings_l": langs,\n'
             u'             "count_postings": 1,\n'
             u'             "count_followers": count_followers,\n'
             u'             "authors": authors,\n'
             u'             "count_mentions": count_mentions});\n'
             u'      }\n'
             u'      this.st_matches.forEach(function(st) {\n'
             u'         emit(st,\n'
             u'            {"count_blog_inlinks": this.blog_inlinks_count,\n'
             u'             "count_microblog_inlinks": this.microblog_inlinks_count,\n'
             u'             "count_postings_l": langs,\n'
             u'             "count_postings": 1,\n'
             u'             "count_followers": count_followers,\n'
             u'             "authors": authors,\n'
             u'             "count_mentions": count_mentions});\n'
             u'      });\n'
             u'   }\n'
             u'}' % search_term_js).encode(u'utf-8'))
    r = Code(u'function(st, values) {\n'
             u'   var count_blog_inlinks = 0;\n'
             u'   var count_microblog_inlinks = 0;\n'
             u'   var count_postings_l = {};\n'
             u'   var count_postings = 0;\n'
             u'   var count_followers = 0;\n'
             u'   var authors = {};\n'            # User name -> number of postings written
             u'   var count_mentions = {};\n'     # User name -> number of @-mentions
             u'   for (var search_term in values) {\n'
             u'      var item = values[search_term];\n'
             u'      for (var lang in item.count_postings_l) {\n'
             u'         if (! search_term) {\n'
             u'            if (isNaN(count_postings_l[lang]))\n'
             u'               count_postings_l[lang] = 0;\n'
             u'            count_postings_l[lang] += item.count_postings_l[lang];\n'
             u'         }\n'
             u'      }\n'
             u'      count_postings += item.count_postings;\n'
             u'      count_followers += item.count_followers;\n'
             u'      for (var key in item.authors) {\n'
             u'         if (isNaN(authors[key]))\n'
             u'            authors[key] = 0;\n'
             u'         authors[key] += item.authors[key];\n'
             u'      }\n'
             u'      for (var key in item.count_mentions) {\n'
             u'         if (isNaN(count_mentions[key]))\n'
             u'            count_mentions[key] = 0;\n'
             u'         count_mentions[key] += item.count_mentions[key];\n'
             u'      }\n'
             u'      if (item.count_blog_inlinks)\n'
             u'         count_blog_inlinks += item.count_blog_inlinks;\n'
             u'      if (item.count_microblog_inlinks)\n'
             u'         count_microblog_inlinks += item.count_microblog_inlinks;\n'
             u'   }\n'
             u'   return {"count_blog_inlinks": count_blog_inlinks,\n'
             u'           "count_microblog_inlinks": count_microblog_inlinks,\n'
             u'           "count_postings_l": count_postings_l,\n'
             u'           "count_postings": count_postings,\n'
             u'           "count_followers": count_followers,\n'
             u'           "authors": authors,\n'
             u'           "count_mentions": count_mentions\n'
             u'          };'
             u'}')

    result_coll = coll.map_reduce(m, r, query=spec)