I'm trying to create a UmbracoApiController to retrieve content from the CMS via ajax using Umbraco 7.1.4

I can make requests to the controller perfectly fine, however the UmbracoHelper throws the following exception when trying to retrieve the AssignedContentItem property:

"Cannot return the IPublishedContent because the UmbracoHelper was constructed with an UmbracoContext and the current request is not a front-end request."

My code is below:

    public class TopicsController : UmbracoApiController
    {
        private IContentService _service;

        public TopicsController(IContentService service)
        {
            _service = service;
        }

        public IEnumerable<Topic> GetTopics()
        {
            return GetTopics(0);
        }

        public IEnumerable<Topic> GetTopics(int topic)
        {
            return _service.GetTopics(Umbraco.AssignedContentItem, topic);
        }
    }

Please can someone tell me why this isn't working? I would like to retrieve content from the CMS via ajax and was able to do this in v6