In service contract management it is possible to sign and lock contract with service amount = zero using event triggers from Codeunits SignServContractDoc and Lock-OpenServContract.

If you create a new Service Contract NOT starting on first day of month, it is also possible to create a Service Contract Invoice for first period with invoice amount = zero. But it is not possible to create further Service Contract Invoice because of checks for invoice amount <> zero.

It would be nice if events for checking zero amount could be implemented in Codeunit ServContractManagement, procedure CreateInvoice (1 check, InvoicedAmount), and Report Create Contract Invoices (2 checks, Amount per Period and InvoicedAmount).