Why does Spring WebClient crash when subscribing to it?

Question

Java

Why does Spring WebClient crash when subscribing to it?
maven :

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
            <version>2.1.3.RELEASE</version>
</dependency>
<dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-core</artifactId>
            <version>3.2.5.RELEASE</version>
 </dependency>

Java :

WebClient client = WebClient.create("http://localhost:8080");
Mono<Employee> employeeMono = client.get()
                .uri("/employees/{id}", "1")
                .retrieve()
                .bodyToMono(Employee.class);

        employeeMono.subscribe(System.out::println);

Full code . The example is taken from here .
Mistake:

io.netty.util.IllegalReferenceCountException: refCnt: 0, decrement: 1
  at io.netty.buffer.AbstractReferenceCountedByteBuf.toLiveRealCnt(AbstractReferenceCountedByteBuf.java:181) ~[netty-buffer-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.buffer.AbstractReferenceCountedByteBuf.release0(AbstractReferenceCountedByteBuf.java:133) ~[netty-buffer-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.buffer.AbstractReferenceCountedByteBuf.release(AbstractReferenceCountedByteBuf.java:124) ~[netty-buffer-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.handler.codec.http.DefaultHttpContent.release(DefaultHttpContent.java:94) ~[netty-codec-http-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.util.ReferenceCountUtil.release(ReferenceCountUtil.java:88) ~[netty-common-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.util.ReferenceCountUtil.safeRelease(ReferenceCountUtil.java:113) ~[netty-common-4.1.33.Final.jar:4.1.33.Final]
  at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:166) [reactor-netty-0.8.5.RELEASE.jar:0.8.5.RELEASE]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:102) [netty-codec-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:438) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:323) [netty-codec-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:297) [netty-codec-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930) [netty-transport-4.1.33.Final.jar:4.1.33.Final]
  at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:799) [netty-transport-native-epoll-4.1.33.Final-linux-x86_64.jar:4.1.33.Final]
  at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427) [netty-transport-native-epoll-4.1.33.Final-linux-x86_64.jar:4.1.33.Final]
  at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328) [netty-transport-native-epoll-4.1.33.Final-linux-x86_64.jar:4.1.33.Final]
  at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905) [netty-common-4.1.33.Final.jar:4.1.33.Final]
  at java.lang.Thread.run(Thread.java:748) [na:1.8.0_201]