graaljs: Cannot assign object data in Java script code. TypeError: writeArrayElement on foreign object failed due to: Message not supported.

Hello

In GraalVM CE 19.2.0, I have jsonObject build in Java using ProxyObject and passed to GraalVM Context. Then I want to update in Java script guest code and got an exception.

...
        //Java on host
        try (Context jsContext = 
                Context.newBuilder("js").allowHostAccess(HostAccess.EXPLICIT).build()) {
                jsContext.getBindings("js").putMember("jsonObject", initDataProxy);
                jsContext.eval("js", jsCode);
            }
        }
...
        //Java script on guest
        for (var key in jsonObject) {
          jsonObject[key] = {'a':'b'};
        }
...

Here exception message:

Exception in thread "main" TypeError: writeArrayElement on foreign object failed due to: Message not supported.
	at <js> :program(Unnamed:1:1557-1583)
	at org.graalvm.polyglot.Context.eval(Context.java:370)
	at com.researchforgood.survey.surveyengine.service.JSProxyBuilderTest.performanceTest(JSProxyBuilderTest.java:174)
	at com.researchforgood.survey.surveyengine.service.JSProxyBuilderTest.main(JSProxyBuilderTest.java:265)

As you see I cannot assign object data in JS code. Am I doing something wrong and this is documented somehow? How can I assign data?