Qt doesn't accept input when configured input method cannot be used

If I configure Qt to use ibus with

export QT_IM_MODULE ibus

and that I quit iBus or that iBus crash, the application doesn't accept input anymore, until I launch iBus again (or that I restart the application).

Qt should use the default input method of the platform as a fallback, but still use iBus as soon as it is restarted. This is the behavior of GTK+.