SDL2 keys F21-F24 are not mapped

I have a special keyboard that can produces keycodes for F21-F24. The SDL2 seems not to hadle it, with the following messages:

The key you just pressed is not recognized by SDL. To help get this fixed, please report this to the SDL forums/mailing list <> X11 KeyCode 199 (191), X11 KeySym 0xFFD2 (F21).

Can’t it be supported somehow?