Steps to Reproduce

<?taglib uri="http://www.zkoss.org/dsp/web/core" prefix="c"?>
<zk>
    <label value="${c:formatNumber(null, '$ ###,###,###.00')}" />
</zk>

Current Result

java.lang.IllegalArgumentException: Cannot format given Object as a Number
java.base/java.text.DecimalFormat.format(DecimalFormat.java:518)
java.base/java.text.Format.format(Format.java:158)
org.zkoss.xel.fn.CommonFns.formatNumber(CommonFns.java:491)
org.zkoss.xel.fn.CommonFns.formatNumber(CommonFns.java:429)

Expected Result

empty string.

Debug Information

• its javadoc mentioned:

  ... If null, an empty string is returned.

• Since it's called on a zul, it's better to return an empty string instead of throwing a runtime exception. An EL expression also evaluate a null as an empty string, no exception.

Workaround

null check