diff --git a/test/lisp/json-tests.el b/test/lisp/json-tests.el
index 05837e83f9..9d7ffd5feb 100644
--- a/test/lisp/json-tests.el
+++ b/test/lisp/json-tests.el
@@ -269,10 +269,13 @@ test-json-encode-plist-with-sort-predicate
     (should (equal (json-encode plist) "{\"a\":1,\"b\":2,\"c\":3}"))))
 
 (ert-deftest test-json-encode-alist-with-sort-predicate ()
-  (let ((alist '((:c . 3) (:a . 1) (:b . 2)))
-        (json-encoding-object-sort-predicate 'string<)
-        (json-encoding-pretty-print nil))
-    (should (equal (json-encode alist) "{\"a\":1,\"b\":2,\"c\":3}"))))
+  (let* ((alist '((:c . 3) (:a . 1) (:b . 2)))
+         (clone (copy-sequence alist))
+         (json-encoding-object-sort-predicate #'string<)
+         (json-encoding-pretty-print nil))
+    (should (equal (json-encode alist) "{\"a\":1,\"b\":2,\"c\":3}"))
+    ;; Ensure sorting isn't destructive (bug#40693).
+    (should (equal alist clone))))
 
 (ert-deftest test-json-encode-list ()
   (let ((json-encoding-pretty-print nil))