Invalid image in signaturefield_json_to_image()

I get about a thousand image-related watchdog errors after trying to save a signature, starting with:

Warning: imagecreatetruecolor(): Invalid image dimensions in signaturefield_json_to_image() (line 21 of /httpdocs/sites/all/modules/contrib/signaturefield/signaturefield.module).

The signature field does not show (broken/missing image).