Ao fazer teste via Postman retorna erro 400

Fala pessoal, estou fazendo o teste do JJWT pelo Postman, porém estou recebendo o erro 400 Bad Request e não estou entendendo o porque. A baixo está o meu código.

TokenService

package br.com.alura.forum.config.security;

import java.util.Date;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.security.core.Authentication;
import org.springframework.stereotype.Service;

import br.com.alura.forum.modelo.Usuario;
import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.SignatureAlgorithm;

@Service
public class TokenService {

    @Value("${forum.jwt.expiration}")
    private String expiration;

    @Value("${forum.jwt.secret}")
    private String secret;

    public String gereToken(Authentication authentication) {
        Usuario logado = (Usuario) authentication.getPrincipal();
        Date hoje = new Date();
        Date dataExpiracao = new Date(hoje.getTime() + Long.parseLong(expiration));

        return Jwts.builder()
                .setIssuer("API do fórum da Alura")
                .setSubject(logado.getId().toString())
                .setIssuedAt(hoje)
                .setExpiration(dataExpiracao)
                .signWith(SignatureAlgorithm.HS256, secret)
                .compact();
    }

}

AutenticacaoController

package br.com.alura.forum.controller;


import javax.validation.Valid;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.ResponseEntity;
import org.springframework.security.authentication.AuthenticationManager;
import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
import org.springframework.security.core.Authentication;
import org.springframework.security.core.AuthenticationException;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import br.com.alura.forum.config.security.TokenService;
import br.com.alura.forum.controller.form.LoginForm;

@RestController
@RequestMapping("/auth")
public class AutenticacaoController {

    @Autowired
    private AuthenticationManager authManager;

    @Autowired
    private TokenService tokenService;

    @PostMapping
    public ResponseEntity<?> autenticar(@RequestBody @Valid LoginForm form){
        UsernamePasswordAuthenticationToken dadosLogin = form.converter();

        try {
            Authentication authentication = authManager.authenticate(dadosLogin);
            String token = tokenService.gereToken(authentication);
            System.out.println(token);
            return ResponseEntity.ok().build();

        } catch (AuthenticationException e) {
            return ResponseEntity.badRequest().build();
        }



    }

}

SecurityConfigurations

package br.com.alura.forum.config.security;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.HttpMethod;
import org.springframework.security.authentication.AuthenticationManager;
import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.builders.WebSecurity;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.config.http.SessionCreationPolicy;

@EnableWebSecurity
@Configuration
public class SecurityConfigurations extends WebSecurityConfigurerAdapter{

    @Override
    @Bean
    public AuthenticationManager authenticationManagerBean() throws Exception {
        return super.authenticationManagerBean();
    }

    //Configurações de autenticação
    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {

    }

    //Configurações de Autorização
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.authorizeRequests()
        .antMatchers(HttpMethod.GET, "/topicos").permitAll()
        .antMatchers(HttpMethod.GET, "/topicos/*").permitAll()    
        .antMatchers(HttpMethod.POST, "/auth*").permitAll()    
        .anyRequest().authenticated()
        .and().csrf().disable()
        .sessionManagement().sessionCreationPolicy(SessionCreationPolicy.STATELESS);
    }


    //Configurações de recursos estaticos(Js,css, Img, etc.)
    @Override
    public void configure(WebSecurity web) throws Exception {

    }

}