meteor exception

exception.txt

Exception from Deps recompute: Error: Can't create second landmark in same branch
    at Object.Spark.createLandmark (http://localhost:3000/packages/spark/spark.js?4af332696fb84f1c71f2e678ad0a267755b2b828:1169:13)
    at http://localhost:3000/packages/templating/deftemplate.js?9369dfa782cc1c8cd4d568925617249ec480b84f:116:24
    at Object.Spark.labelBranch (http://localhost:3000/packages/spark/spark.js?4af332696fb84f1c71f2e678ad0a267755b2b828:1114:14)
    at Object.Meteor._def_template.partial [as row] (http://localhost:3000/packages/templating/deftemplate.js?9369dfa782cc1c8cd4d568925617249ec480b84f:115:22)
    at http://localhost:3000/packages/handlebars/evaluate.js?742a34cf3479d95600582786665b53c0eba4f7d9:352:48
    at Object.Spark.labelBranch (http://localhost:3000/packages/spark/spark.js?4af332696fb84f1c71f2e678ad0a267755b2b828:1114:14)
    at branch (http://localhost:3000/packages/handlebars/evaluate.js?742a34cf3479d95600582786665b53c0eba4f7d9:311:20)
    at template (http://localhost:3000/packages/handlebars/evaluate.js?742a34cf3479d95600582786665b53c0eba4f7d9:351:20)
    at Array.forEach (native)
    at Function._.each._.forEach (http://localhost:3000/packages/underscore/underscore.js?6d71e6711255f43c0de8090f2c8b9f60534a699b:79:11)

index.js

Handlebars.registerHelper('each_with_previous', function(cursor, fn) {
  var previous;
  return cursor.map(function(item) {
    return fn({
      previous: previous,
      item: previous = item
    });
  });
});

Template.messages.beingViewed = function() {
  var pageSize = 100;
  var page = Session.get('page');
  return messages.find({}, {limit: pageSize, offset: pageSize * page});
};

template.hbs

<template name="messages">
  <table>
      <tbody>
        {{#each_with_previous beingViewed}}
          {{> row}}
        {{/each_with_previous}}
      </tbody>
    </table>
</template>

<template name="row">
  <tr>
    <td>{{item.timestamp}}</td>
    <td>{{item.name}}</td>
    <td>{{item.message}}</td>
  </tr>
</template>

i'm trying to iterate through the results of a collection.find() call in a template (like with {{#each}}), but i'm going to want to be able to check the previous row in the row template, so i'm trying to make this each_with_previous template helper work.