SimpleConstraint created by flag causes JS error

<zk>
  <zscript><![CDATA[
    Constraint constraintByString = new SimpleConstraint("no empty");
    Constraint constraintByFlag = new SimpleConstraint(SimpleConstraint.NO_EMPTY);
  ]]></zscript>
  working constraint <datebox constraint="${constraintByString}" />
  <separator/>
  failing constraint <datebox constraint="${constraintByFlag}" /> (leaving empty produces JS error)
</zk>

	<script><![CDATA[
	zk.afterLoad('zul.inp', function() {
		var xSimpleConstraint = {};
		zk.override(zul.inp.SimpleConstraint.prototype, xSimpleConstraint, {
			reparseConstraint : function() {
				if(this._cst) {
					return xSimpleConstraint.reparseConstraint.apply(this, arguments);
				}
			}
		});//zk.override
	});//zk.afterLoad
	]]></script>