Negative joint index leads to crashes

If I need to invalidate a Joint node, I set the index to -1 to be sure that it doesn't override another, valid Joint. Sometimes this change crashes the application with error:
ASSERT failure in QList::operator[]: "index out of range"

To reproduce run the attached example. When the animation is finished, the joint index is changed to -1 (SimpleSkinning.qml line 109). The application crashes.