TCPDF ERROR: Could not include font definition file: courier

I am getting the following error under multiple conditions, but all seem to be related to invoices.

I get this when generating the invoice immediately after signup as part of the checkout process, and I get it when I click the "View PDF" or "Download PDF" buttons on the Admin Invoices page.

The only place I can find "courier" mentioned is in the define for PDF_FONT_MONOSPACED in class.tcpdf.php.

My invoicepdf.tpl makes no reference to "courier" or PDF_FONT_MONOSPACED. It has several
Oh, and General Settings -> Invoices -> TCPDF Font Family is set to Helvetica.

I'm totally baffled (and a bit frustrated). Any ideas where I should look to find out why I'm getting this error?

Thanks in advance.