Issues with a feedback loop.

[jmgelba]

if !but_0 then
        if SETPOINT>0 then
            SETPOINT=SETPOINT-1
           endif
    endif
    if !but_1 then
        if SETPOINT<19 then
            SETPOINT=SETPOINT+1
        endif
    endif

While this generally works, if you reduce SETPOINT to 0, when you let go of the button, SETPOINT increases to 1; how do I fix it so that it stays at 0?