When using the <s:convertEntity /> tag along with a component such as primefaces autocomplete the EntityConverter will throw a NumberFormatException when it is passed an empty string as JSF is prone to do.

<pf:autoComplete
id="fromUserAC"
value="#

"
widgetVar="fromUserAC"
forceSelection="true"
completeMethod="#

"
var="_user"
itemValue="#{_user}"
itemLabel="#{_user.name}"
selectListener="#

"
onSelectUpdate="fromUser"
required="true">
<s:convertEntity />
</pf:autoComplete>

The solution is to add a .isEmpty() check to the converter.