No converter found for return value of type: class java.util.LinkedHashMap

When Prometheus is running from this demo a strange error appears in the log. Looks like a 404 is some specific accept-header might be triggering it.

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.IllegalArgumentException: No converter found for return value of type: class java.util.LinkedHashMap
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:728) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:472) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:395) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:316) ~[tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:395) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:254) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:177) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_144]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_144]
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.29.jar!/:8.5.29]
	at java.lang.Thread.run(Thread.java:748) [na:1.8.0_144]
Caused by: java.lang.IllegalArgumentException: No converter found for return value of type: class java.util.LinkedHashMap
	at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:187) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:208) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81) ~[spring-web-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:113) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.15.BUILD-SNAPSHOT.jar!/:4.3.15.BUILD-SNAPSHOT]
	... 25 common frames omitted