I am unable to write my signature using the trackpad in Preview.app

I stumbled across getting it working for the first time ever. I've tried using it several times with no success. What was different this time, was while the signature window was active but I hadn't yet clicked "Click here to begin", I accidentally clicked the 'text entry' tool that creates a textbox with default red ink. While that was active and accepting input, I then clicked "Click here to begin", and to my surprise it registered my trackpad signature.

Since I couldn't press a key to end the drawing (since the text box was still active and typing my keypresses), I canceled, exited textbox entry, and tried to register a signature again. This time it also worked.

I had the same problem on 10.13.3 High Sierra. Rebooting the computer fixed the issue.