diff --git a/src/json.c b/src/json.c
index 689f6ac510e..c1daba199c3 100644
--- a/src/json.c
+++ b/src/json.c
@@ -352,7 +352,12 @@ lisp_to_json_toplevel_1 (Lisp_Object lisp, json_t **json)
             /* We can't specify the length, so the string must be
                null-terminated.  */
             check_string_without_embedded_nulls (key);
-            int status = json_object_set_new (*json, SSDATA (key),
+            const char *key_str = SSDATA (key);
+            /* Reject duplicate keys.  These are possible if the hash
+               table test is not `equal'.  */
+            if (json_object_get (*json, key_str) != NULL)
+              wrong_type_argument (Qjson_value_p, lisp);
+            int status = json_object_set_new (*json, key_str,
                                               lisp_to_json (HASH_VALUE (h, i)));
             if (status == -1)
               /* FIXME: A failure here might also indicate that the
diff --git a/test/src/json-tests.el b/test/src/json-tests.el
index 9884e9a2d57..5d9f6b3840c 100644
--- a/test/src/json-tests.el
+++ b/test/src/json-tests.el
@@ -52,6 +52,14 @@
     (should (equal (json-serialize table)
                    "{\"abc\":[1,2,true],\"def\":null}"))))
 
+(ert-deftest json-serialize/object-with-duplicate-keys ()
+  (skip-unless (fboundp 'json-serialize))
+  (let ((table (make-hash-table :test #'eq)))
+    (puthash (copy-sequence "abc") [1 2 t] table)
+    (puthash (copy-sequence "abc") :null table)
+    (should (equal (hash-table-count table) 2))
+    (should-error (json-serialize table) :type 'wrong-type-argument)))
+
 (ert-deftest json-parse-string/object ()
   (skip-unless (fboundp 'json-parse-string))
   (let ((input