When I hit 'save' I get this error. If I build or rebuild, then I don't get it.

When I hover over the red lines I see this: "is missing argument 1 of type str"
Here is the method:
public CustInvoiceAccount parmInvoiceAcct(CustInvoiceAccount _InvoiceAccount = InvoiceAccount)
{
InvoiceAccount = _InvoiceAccount;
return InvoiceAccount;
}
Does anyone know the cause?
Thanks