Exporter always return null if related sling model is adapted from SlingHttpServletReqeust

@Model(adaptables = SlingHttpServletRequest.class,
       adapters = TestModel.class,
       resourceType = "core/wcm/components/test")
@Exporter(name = "jackson",
          extensions = "json")
public class TestModel {

    @ValueMapValue
    private String test;

    public String getTest() {
        return test;
    }
}