title: 'producer' type is unknown to ReactiveAdapterRegistry (WebFlux handler)
body: <p>I have the following route in my application</p>
  <pre class="lang-java prettyprint-override"><code>@Bean
  public RouterFunction&lt;ServerResponse&gt; route(UserHandler handler) {
      return RouterFunctions.route(RequestPredicates.POST(&quot;/users&quot;), handler::signup);
  }
  </code></pre>
  <p>which is handled by the following method</p>
  <pre class="lang-java prettyprint-override"><code>public Mono&lt;ServerResponse&gt; signup(ServerRequest request) {
      return request
              .bodyToMono(User.class)
              .map(user -&gt; {
                  user.setPassword(encoder.encode(user.getPassword()));
                  return user;
              })
              .flatMap(repository::save)
              .map(user -&gt; tokenService.create(user.getId()))
              .map(TokenDto::new)
              .flatMap(tokenDto -&gt; ServerResponse.ok().body(tokenDto, TokenDto.class));
  }
  </code></pre>
  <p>When I issue a valid request to the endpoint, the following error is logged</p>
  <pre><code>'producer' type is unknown to ReactiveAdapterRegistry
  </code></pre>
  <p>Having investigated the issue with a debugger, I see that my handler's signup method completes successfully, so I'm assuming the router doesn’t like the return value, but I'm not sure why?</p>