PERIL_WEBHOOK_SECRET is not set up right

Getting this error on the advanced tab of our Github app after having copied the `PERIL_WEBHOOK_SECRET` from it to our heroku instance of Peril.

Is there anything else that could be missing, or causing this?
Afaict from the docs I've copied the right secret to the right ENV var, but I'm still getting this message.

Unable to get it to auth correctly but everything seems like it's right.
I've got a public repo for my settings file, I _think_ it's able to access the settings JSON there ok.
I've got a heroku peril app with the right buildpacks that I keep restarting after any ENV change.

Any thoughts?