Error on hover: lllegal argument, contents must be defined

v0.4.0

in VSCode, create a sample project and add the file

package com.aeschli.testapp;

public class TestHelper {
    public static boolean compare(String a, String b) {
        return a.compareToIgnoreCase(b) == 0;
    }
}

move the mouse over boolean
the dev tools console shows

    at new e (/usr/share/code-insiders/resources/app/out/vs/workbench/node/extensionHostProcess.js:4:227228)
    at asHover (/home/aeschli/.vscode-insiders/extensions/redhat.java-0.4.0/node_modules/vscode-languageclient/lib/protocolConverter.js:63:16)
e.onUnexpectedError @ workbench.main.js:29

The problem seems to be that the hover object sent across the line doesn't contain a 'contents' field.