Regression, can't assign "undefined" to a QML property

This was seen with QML compiled from qt/qt-qml with HEAD at
5eb8e749bdd60ce93737b96f8f736796dfa77281

This code

[...]
    property var returnMessage;
    property var rejectDrop: false;

    function _reset() {
        returnMessage = undefined;    // reset it
        rejectDrop = false;
    }

now result in this error
tesla/common/dragndrop/DragAndDropArea.qml:13: Error: Cannot assign [undefined] to QVariant
this used to work just the other day. Changing the line to

returnMessage = null;    // reset it

fix the problem, but I think the former should still work and that this is a regression.