Authentication with Spring Security + Spring data + MongoDB

I want to use Spring security with MongoDB (using Spring data).

This is my UserService class:

public class UserService {
    private ApplicationContext applicationContext;
    private MongoOperations mongoOperations;

    public UserService() {
        applicationContext = new AnnotationConfigApplicationContext(MongoConfig.class);
        mongoOperations = (MongoOperations) applicationContext.getBean("mongoTemplate");
    }

    public User find(String username) {
        return mongoOperations.findOne(Query.query(Criteria.where("username").is(username)), User.class);
    }
}

And my SecurityConfig class:

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
    @Autowired
    UserService userService;

    @Autowired
    public void configAuthBuilder(AuthenticationManagerBuilder builder) throws Exception {
        builder.userDetailsService(userService); //THIS DOES NOT WORK
        builder.inMemoryAuthentication().withUser("username").password("password").roles("USER");
    }

}

The line I commented says:

The inferred type UserService is not a valid substitute for the bounded parameter <T extends UserDetailsService>.

How can I fix it so I can use MongoDB with Spring Security?