Throwing Error: Argument '_number' is incompatible with the required type.

[pawan]

I have written the code for updating Purchase Order Invoice in my customized Form in write method to update from that customized Form

     PurchFormLetter_Invoice     invoice;
  
   
    ;
    super();

    purchTable = PurchTable::find(nNS_CITPickUpReconcil.PONumber);
    invoice = PurchFormletter::construct(DocumentStatus::Invoice);
    invoice.update(purchtable,
    SystemDateGet(),
    PurchUpdate::All,
    AccountOrder::None,
    false,
    true);

But its throwing error as below:
Argument '_number' is incompatible with the required type.