Angular checkboxes error

I have a little problem with checkboxes :

I have an array of objet. I do a ng-repeat on this array and i associate checkboxes on each element.

<li ng-repeat="title in treeZone track by $index">

<input type="checkbox" ng-model="title.selected" ng-click="functionAdd($index, title.selected)"/> 

</li>

My problèm is that i would like to get all the selected checkboxes with a funciton.

Do do it, i wrote that :

scope.funtionAdd = function() {

          scope.tree_array = "";

          angular.forEach(scope.treeZones, function(title) {

            if (title.selected) { 

              scope.tree_array = scope.tree_array  + title.name + " ";

            }

          });

        }

This function works well but it cause that error : "Property 'selected' does not exist on type 'Zone'" due to "title.selected"

Knowing that my Zone object have 3 properties : id, name, parent.

How resolv this issue knowing that my only way to know if a Zone is selected, is with " title.selected " .

Thanks for help :)