Craft throwing errors with webmentions

[noahread]

I noticed a couple months ago that webmentions stopped working. While testing today via curl I noticed that the server is returning a 500 error:

HTTP/1.1 500 Internal Server Error Server: nginx/1.11.9 Date: Sat, 26 Aug 2017 20:44:31 GMT Content-Type: text/html; charset=utf-8 Transfer-Encoding: chunked Connection: keep-alive Set-Cookie: CraftSessionId=kuqc7b21gqqn0k7i2ju9pjmttr; path=/; secure; HttpOnly Expires: Thu, 19 Nov 1981 08:52:00 GMT Cache-Control: no-store, no-cache, must-revalidate Pragma: no-cache X-Powered-By: Craft CMS Set-Cookie: _ia=ba5da667-79fb-4ab2-a665-72e011278c15; expires=Mon, 26-Aug-2019 20:44:31 GMT; Max-Age=63072000; path=/ charset: utf-8

The Craft logs are showing a php error as well:

Undefined variable: entry (/home/forge/noahread.net/craft/plugins/webmention/services/WebmentionService.php:400)

I’m not sure where to look next. Any thoughts?