Cannot get upickle working with scala.js in a scalatra servlet - scalatra

Question

When I try calling upickle.write from anywhere in a servlet I get an exception. If for example I do
upickle.write("x")
I get the exception below. Any suggestions?
java.lang.ClassCastException: java.lang.String cannot be cast to scala.scalajs.js.Any
at upickle.json.package$.write(package.scala:38)
at upickle.Types$class.write(Types.scala:127)
at upickle.package$.write(package.scala:10)
at MyServlet$$anonfun$5.apply(MyServlet.scala:157)
at org.scalatra.ScalatraBase$class.org$scalatra$ScalatraBase$$liftAction(ScalatraBase.scala:270)
at org.scalatra.ScalatraBase$$anonfun$invoke$1.apply(ScalatraBase.scala:265)
at org.scalatra.ScalatraBase$$anonfun$invoke$1.apply(ScalatraBase.scala:265)
at org.scalatra.ScalatraBase$class.withRouteMultiParams(ScalatraBase.scala:341)
at org.scalatra.ScalatraServlet.withRouteMultiParams(ScalatraServlet.scala:49)
at org.scalatra.ScalatraBase$class.invoke(ScalatraBase.scala:264)
at org.scalatra.ScalatraServlet.invoke(ScalatraServlet.scala:49)
at org.scalatra.ScalatraBase$$anonfun$runRoutes$1$$anonfun$apply$8.apply(ScalatraBase.scala:240)
at org.scalatra.ScalatraBase$$anonfun$runRoutes$1$$anonfun$apply$8.apply(ScalatraBase.scala:238)
at scala.Option.flatMap(Option.scala:171)
at org.scalatra.ScalatraBase$$anonfun$runRoutes$1.apply(ScalatraBase.scala:238)
at org.scalatra.ScalatraBase$$anonfun$runRoutes$1.apply(ScalatraBase.scala:237)
at scala.collection.immutable.Stream.flatMap(Stream.scala:493)
at org.scalatra.ScalatraBase$class.runRoutes(ScalatraBase.scala:237)
at org.scalatra.ScalatraServlet.runRoutes(ScalatraServlet.scala:49)
at org.scalatra.ScalatraBase$class.runActions$1(ScalatraBase.scala:163)
at org.scalatra.ScalatraBase$$anonfun$executeRoutes$1.apply$mcV$sp(ScalatraBase.scala:175)
at org.scalatra.ScalatraBase$$anonfun$executeRoutes$1.apply(ScalatraBase.scala:175)
at org.scalatra.ScalatraBase$$anonfun$executeRoutes$1.apply(ScalatraBase.scala:175)
at org.scalatra.ScalatraBase$class.org$scalatra$ScalatraBase$$cradleHalt(ScalatraBase.scala:193)
at org.scalatra.ScalatraBase$class.executeRoutes(ScalatraBase.scala:175)
at org.scalatra.ScalatraServlet.executeRoutes(ScalatraServlet.scala:49)
at org.scalatra.ScalatraBase$$anonfun$handle$1.apply$mcV$sp(ScalatraBase.scala:113)
at org.scalatra.ScalatraBase$$anonfun$handle$1.apply(ScalatraBase.scala:113)
at org.scalatra.ScalatraBase$$anonfun$handle$1.apply(ScalatraBase.scala:113)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
at org.scalatra.DynamicScope$class.withResponse(DynamicScope.scala:80)
at org.scalatra.ScalatraServlet.withResponse(ScalatraServlet.scala:49)
at org.scalatra.DynamicScope$$anonfun$withRequestResponse$1.apply(DynamicScope.scala:60)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
at org.scalatra.DynamicScope$class.withRequest(DynamicScope.scala:71)
at org.scalatra.ScalatraServlet.withRequest(ScalatraServlet.scala:49)
at org.scalatra.DynamicScope$class.withRequestResponse(DynamicScope.scala:59)
at org.scalatra.ScalatraServlet.withRequestResponse(ScalatraServlet.scala:49)
at org.scalatra.ScalatraBase$class.handle(ScalatraBase.scala:111)
at org.scalatra.ScalatraServlet.org$scalatra$servlet$ServletBase$$super$handle(ScalatraServlet.scala:49)
at org.scalatra.servlet.ServletBase$class.handle(ServletBase.scala:43)
at MyServlet.org$scalatra$scalate$ScalateSupport$$super$handle(OutrightMarketServlet.scala:146)
at org.scalatra.scalate.ScalateSupport$class.handle(ScalateSupport.scala:130)
at MyServlet.handle(MyServlet.scala:146)
at org.scalatra.ScalatraServlet.service(ScalatraServlet.scala:54)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:669)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:455)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:560)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1072)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:382)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1006)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
at org.eclipse.jetty.server.Server.handle(Server.java:365)
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:485)
at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:926)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:988)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:640)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:628)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
at java.lang.Thread.run(Thread.java:745)