Form Validation - Check That Only Numbers Entered

I am using this code to validate a form to check that only numbers have been entered but when I enter alphabets the javascript raises an error but still submits the form.

<script type='text/javascript'>
function isNumeric(elem, helperMsg){
var numericExpression = /^[0-9]+$/;
if(elem.value.match(numericExpression)){
return true;
}else{
alert(helperMsg);
elem.focus();
return false;
}}
</script>
<form>
Numbers Only: <input type='text' id='numbers' action= "desktop.html"/>
<input type='button'
onclick="isNumeric(document.getElementById('numbers'), 'Numbers Only Please')"
value='Check Field' />
</form>