Help figure out the source of a `file-notify--call-handler` error

At random points during the day I see the following error message: file-notify--call-handler: Symbol’s function definition is void Unfortunately, I cannot figure out what the source of this is. I've tried all sorts of things, including (setq debug-on-message "file-notify--call-handler: Symbol’s function definition is void") and setting `debug-on-error` to `t`. But I keep getting these messages (overnight, the message popped up about 29 times, according to the messages buffer) with no explanation. Any suggestions on what else I can try here? **EDIT**: I realized I failed to add the last bit of the message. It actually reads: file-notify--call-handler: Symbol’s function definition is void: nil

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Does `C-h f file-notify--call-handler` work?

When you set `debug-on-error`, did you get a `*Backtrace*` buffer showing you the stack that caused it? Probably the easiest thing is to grep all the Lisp loaded into your Emacs for `file-notify--call-handler`. That function is part of the built-in `filenotify.el`, so likely something you're using is a) using internal stuff in `filenotify` and b) not `require`ing it.

OK, so maybe I should start with a question. What should I make of the error `file-notify--call-handler: Symbol’s function definition is void: nil`? Is it telling me that `file-notify--call-handler` is undefined? (I now realize I forgot to add the end of the message...) At any rate, if I grep all the lisp in my load-path for `file-notify--call-handler` I get nothing, except in `savehist.el` (`mini-buffer-history`).

Yes, it does. ¯\_(ツ)_/¯

> OK, so maybe I should start with a question. What should I make of the error `file-notify--call-handler: Symbol’s function definition is void: nil`? Is it telling me that `file-notify--call-handler` is undefined? (I now realize I forgot to add the end of the message...) It's saying that `nil`'s function definition is void, and this error happened somewhere inside `file-notify--call-handler`. So you need to track down anything using filenotify to figure out which one has a buggy callback.