Ember Nested URLs Error

post-route.js

// ...

  model: function(params) {

    // ...
    
    var versions = _this.store.findRecord('post-version', post.get('id')); 
    
    // http://localhost:4101/api/v1/posts/1/versions gets called correctly...
    
    // ... but then BOOM. "Cannot read property '_internalModel' of undefined"
    
    // ...

  }

// ...

post-version-adapter.js

import DS from "ember-data";

export default DS.RESTAdapter.extend({
  host: 'http://localhost:4101/api/v1',
  buildURL: function(root, suffix, record) {
    var url = this._super();
    return url + '/posts/' + suffix + '/versions';
  }
});

post-version-model.js

import DS from 'ember-data';

export default DS.Model.extend({
  post_id: DS.attr('number'),
  version: DS.attr('number'),
  title: DS.attr('string'), 
  body: DS.attr('string')
});

response.json

{
  "post_versions": [
    {"id":13,"post_id":65,"version":1,"title":"test title", "body":"test body"}
  ]
}