Javascript error notice not showing every time

Making a simple converter for convert a integer (1 - 99) to time in (1 - 60 minutes)

The formula works perfect, and converts 'on-the-fly', but i have problems with the error handling. When someone type '0' the first time after the webpage loaded then shows a notice, but when after a few converts and type '344' then the notice doens't showing anymore.

How can show the notice every time a number <= 0 OR number >= 100 OR isNaN(number) ?

The Javascript code:

<script>
function MinutesConverter(valNum) {
    var x, notice, displayminutes
    x = document.getElementById("inputnumber").value;
    notice = document.getElementById('notice');
    displayminutes = document.getElementById('minutes');
    outputMinutes = document.getElementById("outputMinutes");

    if ( isNaN(x) || x <= 0 || x >= 100 ) {
        notice.innerHTML = "Voer een geheel getal in tussen 0 en 100";
  } else {
        notice.style.display="none";
        displayminutes.innerHTML="minuten";
        outputMinutes.innerHTML="";
    outputMinutes.innerHTML=Math.round((valNum/100)*60);
  }
}
</script>

And that's the experimental static webpage.