Cannot change string values when stepping in .js-file

1. Open qtdeclarative/examples/quick/demos/samegame/samegame.qmlproject from Qt5's repository.
   The problem will be the same if you open the .pro-file instead.
2. Place a breakpoint at the first line of the startNewGame() function (samegame.js) which contains executable code (i.e. not (only) a comment).
3. Start debugging app
4. In the app, click "1 Player".
   The breakpoint is being hit.
5. Try to change the value of "this->gameMode" in the Locals and Expressions view.
   It does not work.

It should be possible to change this value. With numeric values this seems to work.