Polymer Ajax: Uncaught TypeError: Cannot set property 'contentType' of undefined -

i keep getting error:"uncaught typeerror: cannot set property 'contenttype' of undefined" on this.$.ajax_grid.contenttype="application/json";

i've found tons of topics such error, seems individual code related :/

<!-- @license copyright (c) 2016 polymer project authors. rights reserved. code may used under bsd style license found @ http://polymer.github.io/license.txt complete set of authors may found @ http://polymer.github.io/authors.txt complete set of contributors may found @ http://polymer.github.io/contributors.txt code distributed google part of polymer project subject additional ip rights grant found @ http://polymer.github.io/patents.txt -->  <link rel="import" href="../../bower_components/polymer/polymer.html">  <dom-module id="my-afmelden-test">    <template>   <iron-ajax       auto       url = "../../signups.json"       handle-as="json"       last-response="{{users}}" >     </iron-ajax>      <vaadin-grid id="grid" items="[[users]]" selected-items="{{selected}}">       <vaadin-grid-selection-column auto-select="[[autoselect]]"></vaadin-grid-selection-column>       <vaadin-grid-column width="50px" flex-grow="0">         <template class="header">#</template>         <template>[[index]]</template>       </vaadin-grid-column>       <vaadin-grid-column width="50px" flex-grow="0">         <template class="header"></template>         <template>           <img src="[[item.user.picture.thumbnail]]"></img>         </template>       </vaadin-grid-column>       <vaadin-grid-column>         <template class="header">first name</template>         <template>           <div class="capitalized">[[item.user.name.first]]</div>         </template>       </vaadin-grid-column>       <vaadin-grid-column>         <template class="header">last name</template>         <template>           <div class="capitalized">[[item.user.name.last]]</div>         </template>       </vaadin-grid-column>       <vaadin-grid-column>         <template class="header">custom selection</template>         <template>         <div style="display: flex; align-items: center">           <paper-button raised on-tap="_deselect" hidden="[[!selected]]">deselect</paper-button>           <paper-button raised on-tap="_select" hidden="[[selected]]">select</paper-button>           <paper-checkbox checked="{{selected}}">selected</paper-checkbox>         </div>         </template>       </vaadin-grid-column>     </vaadin-grid> <paper-button on-tap="_myafmelden" raised>afmelden</paper-button> <paper-button on-tap="_exportjson" raised>export json</paper-button> <paper-button on-tap="_exportcsv" raised>export csv</paper-button> </template>  <iron-ajax   id="ajax_grid"   method="post"   url="/cgi-bin/gerecht-toevoegen.py"   handle-as="json"   on-response="_myafmelden_responsehandler"> </iron-ajax>  </template>    <script>     polymer({       is: 'my-afmelden-test',        properties: {           selected: {             type: array,           },         },        _myafmelden: function() {         console.log("derpdiedierp");         this.$.ajax_grid.contenttype="application/json";         this.$.ajax_grid.body={             "selection":this.selected         };         this.$.ajax_grid.generaterequest();       },        _myafmelden_responsehandler: function(request) {         console.log("response: " + request_confirm);       },       _exportjson: function() {         console.log(json.stringify(this.selected));       },       // _exportcsv: function() {       //   console.log(this.selected.map(p => `"${p.name.first}", "${p.name.last}"`).join("\n"));       // }     });   </script> </dom-module>  <!-- https://cdn.vaadin.com/vaadin-grid/2.0.0-beta1/demo/selection.html -->