[GNC] Printing Invoices

[Steve Brown]

File->Print Invoice prints an opened invoice using the template at
gnucash/report/reports/standard/invoice.scm.

So does the Print Invoice button in the Find Invoice screen
And ditto the Print Invoice in the heading of the View Invoice screen

OTOH, Reports->Business->Printable Invoice just gives a "No valid
invoice selected" message. The other Reports->Business invoice reports
do the same thing.

The wiki gives some instructions to avoid this, but they don't seem to
work. 

In looking at the scheme file, this is caused by an "invoice" argument
of null being passed.

What am I missing?

I'm trying to create a custom report with 4.13