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. ----------