adapter.js

DS.RESTAdapter.reopen({
  fetchBatchSize: 25,
  findMany: function(store, type, ids, owner){
    var root = type.typeKey,
        batchSize = this.get('fetchBatchSize') || ids.length,
        batch,
        rest = ids,
        promises = [],
        key = this.pathForType(root);

    while(rest.length > 0){
      batch = rest.slice(0, batchSize);
      rest = rest.slice(batchSize);
      promises.push(this.ajax(this.buildURL(root), "GET", {
        data: { ids: batch }
      }));
    }

    return Ember.RSVP.all(promises).then(function(results){
      return results.reduce(function(acc, item){
        acc[key] = acc[key].concat(item[key]);
        return acc;
      });
    });
  },
  pathForType: function(type){
    return type.pluralize().underscore();
  }
});

error.md

Assertion failed: The response from a findMany must be an Array, not undefined

Here's the line it points to (the assertion):

function _findMany(adapter, store, type, ids, owner) {
  var promise = adapter.findMany(store, type, ids, owner),
      serializer = serializerForAdapter(adapter, type);

  return resolve(promise, "DS: Handle Adapter#findMany of " + type).then(function(payload) {
    payload = serializer.extract(store, type, payload, null, 'findMany');

    Ember.assert("The response from a findMany must be an Array, not " + Ember.inspect(payload), Ember.typeOf(payload) === 'array');

    store.pushMany(type, payload);
  }, null, "DS: Extract payload of " + type);
}