Expected array for `items` polymer in my console

Question

I'm learning Polymer 1.8 and I'm trying to get data from an api. after that i want to bind data from my ontap() function with a dom-repeate, but I'm getting a weird error in the console: [dom-repeat::dom-repeat]: expected array for items, found [{"requestnum":"1045","orgid":"EAGLENA"},{"requestnum":"1046","orgid":"EAGLENA"},{"requestnum":"1047","orgid":"EAGLENA"},{"requestnum":"1048","orgid":"EAGLENA"},{"requestnum":"1049","orgid":"EAGLENA"}]

i can't solve this problem could any one help me? What is wrong in this code????
im using polymer 1.8

*this is my html file maximo-test-card.html:

    <dom-module id="maximo-test-card">  
      <template>

            <template is="dom-repeat" items="{{hi}}">
                <li>
                    <span>{{item.requestnum}}</span>

                </li>
            </template>


    <div>{{hi}}</div>

            <iron-ajax
            id="requestRepos"
            headers='{"Content-Type": "application/json", "Accept": "someAccept", "Authorization": "someAuthorizationToken"}'
            url="http://vaganet.vaganet.fr:9080/maximo/oslc/os/mxinvres?oslc.select=*" 

            handle-as="json"
            on-response="ontap"> </iron-ajax> </template>
     <script src="maximo-test-card.js"></script>

    </dom-module>

*this is my js file maximo-test-card.js:  
    Polymer({
        is: 'maximo-test-card',


        properties: {
            repos: {
                type: Array
            },
            githubrepository:{
              type: Array
            },

            hi:{
                type: Array,

            },


        },

        ready: function () {
    this.$.requestRepos.generateRequest();
            //console.log(hi)
            //this.name=hi
        },

        ontap: function (data) {
            this.repos = data.detail.response.member;

            hi = []
            for (var i = 0; i < 5; i++) {
                hi[i] = {"requestnum":this.repos[i].requestnum,"orgid":this.repos[i].orgid}
            }
            this.hi = JSON.stringify(hi)
            alert(this.hi);
            return this.hi;

          },

    });