angular link scope variable undefined into template

I have defined a directive that uses ui.calendar as sub directive:

angular.module('calendar.availability')
.directive('calendarAvailability', ['$http','$modal','$log', function($http,$modal,$log) {
        return {
        restrict: 'E'
        , replace: true
        , template: '<div><div ui-calendar="uiConfig.calendar" ng-model="eventsources"></div></div>'
        , scope: {
             availability: '=',
             delegation: '='
        }
        , link: function($scope, elm, attrs, ctrl) {
            $scope.eventsources=[];
            var removeAvailabilitySkeleton = function() {
                $scope.eventsources[0] = [];
            };
            $scope.$watch('availability', function(n, o) {
               if (n == o) return;
               removeAvailabilitySkeleton();
               if (n == 0) return;
               $http.get('ws/availability.php', {params: {doctor: $scope.availability,delegation: $scope.delegation}})
                .success(function(xhr) {
                $scope.eventsources[0] = xhr;
               });
            });
            //........... event handlers

            $scope.uiConfig = {
            calendar: {
                editable: true,
                selectable: true,
                selectHelper: true,
                weekends: false,
                defaultView: 'agendaWeek',
                unselectCancel: '.eventeditor',
                allDaySlot: false,
                slotEventOverlap: false,
                dayNames: ['domingo', 'lunes', 'martes', 'miercoles', 'jueves', 'viernes', 'sabado'],
                dayNamesShort: ['dom', 'lun', 'mar', 'mie', 'jue', 'vie', 'sab'],
                header: {
                left: 'month agendaWeek agendaDay',
                center: 'title',
                right: 'today prev,next'
                },
                dayClick: $scope.alertEventOnClick,
                eventDrop: $scope.alertOnDrop,
                eventResize: $scope.alertOnResize,
                select: $scope.selectRange
            }
            };
        }

        }
    }])

the problem is that, at render time, the sub directive ui.calendar don't find the ng-model eventsources (is undefined) and throws error. Any help is apreciated