diff --git a/lisp/outline.el b/lisp/outline.el
index 9058293e277..8dffca756fc 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -281,8 +281,8 @@ buttons should look."
   :version "29.1")
 
 (defcustom outline-minor-mode-buttons
-  '(("▶️". "🔽")
-    ("▶" . "▼"))
+  '(("▶️" "🔽" outline--valid-emoji-p)
+    ("▶" "▼" outline--valid-char-p))
   "List of close/open pairs to use if using buttons."
   :type 'sexp
   :version "29.1")
@@ -951,14 +951,20 @@ If non-nil, EVENT should be a mouse event."
   (outline-flag-subtree t))
 
 (defun outline--make-button (type)
-  (cl-loop for (close . open) in outline-minor-mode-buttons
-           when (and (char-displayable-p (aref close 0))
-                     (char-displayable-p (aref open 0)))
+  (cl-loop for (close open test) in outline-minor-mode-buttons
+           when (and (funcall test close) (funcall test open))
            return (concat (if (eq type 'close)
                               close
                             open)
                           " " (buffer-substring (point) (1+ (point))))))
 
+(defun outline--valid-emoji-p (string)
+  (when-let ((font (car (internal-char-font nil ?😀))))
+    (font-has-char-p font (aref string 0))))
+
+(defun outline--valid-char-p (string)
+  (char-displayable-p (aref string 0)))
+
 (defun outline--make-button-overlay (type)
   (let ((o (seq-find (lambda (o)
                        (overlay-get o 'outline-button))