Add-PWPDFDigitalSignature - not working

When I use Add-PWPDFDigitalSignature I get an error

Add-PWPDFDigitalSignature : Value cannot be null.

The error occurs after a Windows Security dialog box shows up asking to Select a Certificate.

My code is simple for testing.

Get-PWDocumentsBySearch -FolderPath 'ABC123\Drawing' -FileName test.pdf | Add-PWPDFDigitalSignature -PDFSignatureFile 'C:\Temp\Signature.pdf'

I tried looking at the examples but one example but they're copied over from Add-PWPDFQRCode

Either I'm missing a step for the certificate or this is a bug.