No message found under code 'hello' for locale 'null' 오류 해결이 안됩니다

package hello.itemservice.message;


import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.context.MessageSource;



import static org.assertj.core.api.Assertions.*;

@SpringBootTest
public class MessageSourceTest {

    @Autowired
    MessageSource ms;

    @Test
    void helloMessage(){

        String result = ms.getMessage("hello", null, null);
        assertThat(result).isEqualTo("안녕");
    }
}

강사님이 알려주신것 처럼 작성했더니 org.springframework.context.NoSuchMessageException: No message found under code 'hello' for locale 'null'. 이런 오류 메시지가 발생합니다.