Call to undefined function get_header() in wp-includes/theme-compat/embed.php:13

Any site of mine (hosted on Pantheon) that has Defender installed, keeps logging this error over and over again. Please see attached. There is also often a similar one about an undefined date. I understand this is a vague error and for a while I was not sure what was causing it, but I then realized started on a new site immediately after activating Defender.

Any ideas?