ConcurrentModificationException when serving Servlet.fn request and Undertow

[dsyer]

I get this with Undertow (but not Tomcat):

Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1461)
	at io.undertow.servlet.util.IteratorEnumeration.nextElement(IteratorEnumeration.java:44)
	at org.springframework.web.servlet.function.DefaultServerRequest$ServletAttributesMap.clear(DefaultServerRequest.java:377)
	at org.springframework.web.servlet.function.RequestPredicates.restoreAttributes(RequestPredicates.java:292)
	at org.springframework.web.servlet.function.RequestPredicates.access$300(RequestPredicates.java:67)

Basic app:

@SpringBootApplication
public class ServletApplication {

	public static void main(String[] args) {
		SpringApplication.run(ServletApplication.class, args);
	}

	@Bean
	public RouterFunction<?> userEndpoints() {
		return route().GET("/hello", request -> ok().body("Hello")).build();
	}

}

The error shows up when you try to visit a non-existent endpoint (e.g. http://localhost:8080/).