Is it possible to run an AsyncWebRequest via browser call?

Question

I'm using Spring Boot 1.4.

@RequestMapping("/nonblocking/{identifier}")
fun get(@PathVariable identifier: String): DeferredResult<List<Quote>> {
    logger.info("Starting call")
    val dr = DeferredResult<List<Quote>>()
    CompletableFuture.supplyAsync({quoteRepository.getAll(identifier)})
            .whenComplete { list, throwable -> dr.setResult(list) }
    logger.info("Finishing call")
    return dr
}

When I try to run 500 call using gatling I get this error after a while:

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.IllegalStateException: Can't set AsyncWebRequest with concurrent handling in progress
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:720) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:468) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:391) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:318) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:445) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:304) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:181) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.connector.CoyoteAdapter.asyncDispatch(CoyoteAdapter.java:388) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.coyote.http11.AbstractHttp11Processor.asyncDispatch(AbstractHttp11Processor.java:1715) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:652) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1502) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1458) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_65]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_65]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_65]
Caused by: java.lang.IllegalStateException: Can't set AsyncWebRequest with concurrent handling in progress
    at org.springframework.util.Assert.state(Assert.java:392) ~[spring-core-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.context.request.async.WebAsyncManager.setAsyncWebRequest(WebAsyncManager.java:108) ~[spring-web-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:813) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.0.RC2.jar:4.3.0.RC2]
    ... 24 common frames omitted

Can't set AsyncWebRequest with concurrent handling in progress

I cannot find any information on this type of error. I was trying to use gatling to run test. Could be that the problem? Because I guess if I was to use correctly a DeferredResult I need to use a client that supports async.