I am unable to write my signature using the trackpad in Preview.app I click on the signature button in the Preview.app toolbar:



The following box pops up:



I click on the Click Here To Begin button. The box is now ready for input:



However, I cannot get any markings at all to show up as my signature. My box always looks like the above image.

I've tried doing the above in an image document and a PDF document. I've tried holding down the trackpad while writing and not holding the trackpad down while writing. I've tried using a mouse.

Is there a fix for this bug, so that I can use my Macbook trackpad to write my signature?

Note: the Camera-based signature method technically works on my machine. However, its rendition ability isn't great.



OS X El Capitan, version 10.11.6.
I had the same problem on 10.13.3 High Sierra.  Rebooting the computer fixed the issue.