如何从API加载数据？

I made this JavaScript (JQuery) to load the data from an API call but it gives me an undefined. This is how it looks:

$(document).ready(function(){
    //Autocomplete for Person
    $(function() {
        $.ajax({
            type: 'GET',
            url: urlReq+'/api/persons/1',
            success: function(response) {
                let modelArray = response;
                let dataModel = {};

                for (let i = 0; i < modelArray.length; i++) {
                    dataModel[modelArray[i].first_name] = null;
                    console.log('data is: '+modelArray[i].first_name);
                }

                $('input.autocomplete_person').autocomplete({
                    data: dataModel,
                });
            }
        });
    });
});

where my urlReq+'/api/persons/1' returns something like this:

{"452":{"id":452,"reference":"20190528155926096","first_name":"John","middle_name":"Smith","third_name":null,"family_name":"Doe"}}

but my problem is that my console gives me this: data is: undefined