Empty array response when returning a JSON

Question

I tried to return a list of records as JSON from the controller as follows:

public class ListsController extends APIController  {

    public void index(){
        respond(List.findAll().toJson(true)).contentType("application/json");
    }
}

ApiController is defines as follows:

public abstract class APIController extends AppController {
    @Override
    protected String getContentType() {
        return "application/json";
    }

    @Override
    protected String getLayout() {
        return null;
    }

}

When I test it:

@Test
    public void shouldRenderLists(){
        app.models.List list = new app.models.List();
        list.set("id", 1);
        list.set("code", "some code");
        list.set("name", "some name");
        list.set("item_code", "some item code");
        list.saveIt();
        request().integrateViews().get("index");

        the(contentType()).shouldBeEqual("application/json");
        Map[] listsMaps = JsonHelper.toMaps(responseContent());

        the(listsMaps.length).shouldBeEqual(1);
        Map listEntry = listsMaps[0];

        the(listEntry.get("code")).shouldBeEqual("some code");
        the(listEntry.get("name")).shouldBeEqual("some name");
    }

It fails because responseContent() returns an [\n\n]. Any ideas why so ?