test 실행시 org.springframework.http.converter.HttpMessageNotWritableException 500 에러 발생합니다.

package com.devyu.webmvc;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.is;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.http.MediaType;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.test.web.servlet.MockMvc;


@RunWith(SpringRunner.class)
@WebMvcTest(UserController.class) 
public class UserControllerTest {

	@Autowired
	MockMvc mockMvc;

	@Test
	public void hello() throws Exception {
		mockMvc.perform(get("/hello"))
						.andExpect(status().isOk())
						.andExpect(content().string("안녕"));
						
	}
	

	@Test
	public void createUser() throws Exception {
		String userJson="{\"userName\":\"devyu\", \"userPassword\":\"min\"}";
		mockMvc.perform(post("/create/user")
				.contentType(MediaType.APPLICATION_JSON_UTF8)
				.accept(MediaType.APPLICATION_JSON_UTF8)
				.content(userJson))
			  .andExpect(status().isOk())
		         .andExpect(jsonPath("$.username",is(equalTo("devyu"))))
		         .andExpect(jsonPath("$.password",is(equalTo("min"))));
	}
}

우선 controller단에 요청하고 @RequestBody와 HttpMessageConverter를 통해 객체에 json값을 주입해주는것 까지는 정상작동 합니다.(해당 controller단에서 user객체를 print() 해보았습니다.) 문제는 객체를 return 해주면 org.springframework.http.converter.HttpMessageNotWritableException 500 에러 발생합니다. 원인이 무엇일까요???