2018.1.1 Spring Boot 2 Tests Jackson Serialization issue HttpMessageNotWritableException

I have made a simple model class 

public final class DomainResponse {

    private String content;
    private Code code;
    private DomainResponse() {

    }

    public static DomainResponse of(final String content, final Code code) {
        DomainResponse dr = new DomainResponse();
        dr.content = content;
        dr.code = code;
        return dr;
    }

    public Code getCode() {
        return code;
    }

    public void setCode(final Code code) {
        this.code = code;
    }
}

Along with a Controller and Service class and have written the following test class:

@RunWith(SpringRunner.class)
@WebMvcTest(DomainController.class)
public class DomainControllerContractTest {

    @MockBean
    private DomainService service;

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void shouldReturnDefaultMessage() throws Exception {
        when(service.serveWithName(any(String.class))).thenReturn(DomainResponse.of("Hello World", Code.OK));
        this.mockMvc.perform(get("/domain")).andDo(print()).andExpect(status().isOk())
                .andExpect(content().string(containsString("{\"code\":\"OK\"}")));
    }
}

when I perform `gradle clean build` in the terminal, the test passes, so the `DomainResponse` class is being serialized

but when I run it in IntelliJ IDEA, the test fails with the exception:

Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class au.com.auspost.myapi.domain.DomainResponse

I imagine that there is some IDE-based implementation of Jackson getting in the way, but I can't find it, can anyone help with this?