xev output:
ButtonPress event, serial 26, synthetic NO, window 0x2600001,
    root 0x136, subw 0x0, time 2489798319, (447,576), root:(448,593),
    state 0x10, button 10, same_screen YES

ButtonRelease event, serial 26, synthetic NO, window 0x2600001,
    root 0x136, subw 0x0, time 2489798455, (447,576), root:(448,593),
    state 0x10, button 10, same_screen YES


code that fails:
    mpress("Button10", "WScreen.switch_next(_)"),
    mpress("Button11", "WScreen.switch_prev(_)"),