diff --git a/core/core-lib.el b/core/core-lib.el
index c1309672d..f5da5a23a 100644
--- a/core/core-lib.el
+++ b/core/core-lib.el
@@ -215,8 +215,20 @@ See `eval-if!' for details on this macro's purpose."
 (defmacro fn! (arglist &rest body)
   "Returns (cl-function (lambda ARGLIST BODY...))
 The closure is wrapped in `cl-function', meaning ARGLIST will accept anything
-`cl-defun' will. "
+`cl-defun' will. Implicitly adds `&allow-other-keys' if `&key' is present in
+ARGLIST."
   (declare (indent defun) (doc-string 1) (pure t) (side-effect-free t))
+  ;; Don't complain about undeclared keys.
+  (when (memq '&key arglist)
+    (if (memq '&aux arglist)
+        (let (newarglist arg)
+          (while arglist
+            (setq arg (pop arglist))
+            (when (eq arg '&aux)
+              (push '&allow-other-keys newarglist))
+            (push arg newarglist))
+          (setq arglist (nreverse newarglist)))
+      (setq arglist (append arglist (list '&allow-other-keys)))))
   `(cl-function (lambda ,arglist ,@body)))
 
 (defmacro cmd! (&rest body)