QuickBooks v3 API error: An application error has occurred while processing your request: System Failure Error: java.lang.NumberFormatException: For input string: ""

Problem:

In this particular case, an invoice had a line item with just a <ItemId />. If you include the ItemId node within an invoice it must be filled in with a value or you need to leave it off completely. If you are getting this error on another transaction or name entity then look for blank nodes.