diff --git a/mysql-test/main/func_json.result b/mysql-test/main/func_json.result
index 716ae78a35c..cc87a935a85 100644
--- a/mysql-test/main/func_json.result
+++ b/mysql-test/main/func_json.result
@@ -1221,3 +1221,14 @@ SELECT JSON_ARRAYAGG(a) FROM t1;
 JSON_ARRAYAGG(a)
 NULL
 DROP TABLE t1;
+#
+# MDEV-21915 Server crashes in copy_fields,Item_func_group_concat::add
+while using json_arrayagg() as a window function
+#
+select json_arrayagg(a) over () from (select 1 a) t;
+ERROR 42000: This version of MariaDB doesn't yet support 'JSON_ARRAYAGG() aggregate as window function'
+select json_objectagg(a, b) over () from (select 1 a, 2 b) t;
+ERROR 42000: This version of MariaDB doesn't yet support 'JSON_OBJECTAGG() aggregate as window function'
+#
+# End of 10.5 tests
+#
diff --git a/mysql-test/main/func_json.test b/mysql-test/main/func_json.test
index 2c5dbfbd42c..216d51b4c36 100644
--- a/mysql-test/main/func_json.test
+++ b/mysql-test/main/func_json.test
@@ -740,3 +740,19 @@ DROP TABLE t1;
 CREATE TABLE t1 (a INT);
 SELECT JSON_ARRAYAGG(a) FROM t1;
 DROP TABLE t1;
+
+-- echo #
+-- echo # MDEV-21915 Server crashes in copy_fields,Item_func_group_concat::add
+-- echo       while using json_arrayagg() as a window function
+-- echo #
+
+--error ER_NOT_SUPPORTED_YET
+select json_arrayagg(a) over () from (select 1 a) t;
+
+--error ER_NOT_SUPPORTED_YET
+select json_objectagg(a, b) over () from (select 1 a, 2 b) t;
+
+--echo #
+--echo # End of 10.5 tests
+--echo #
+
diff --git a/sql/item_jsonfunc.h b/sql/item_jsonfunc.h
index 44f9e8146c2..f1afaf6cc31 100644
--- a/sql/item_jsonfunc.h
+++ b/sql/item_jsonfunc.h
@@ -579,7 +579,7 @@ public:
   bool is_json_type() { return true; }
   void cleanup();
 
-  enum Sumfunctype sum_func () const {return GROUP_CONCAT_FUNC;}
+  enum Sumfunctype sum_func () const {return JSON_OBJECTAGG_FUNC;}
   const char *func_name() const { return "json_objectagg("; }
   const Type_handler *type_handler() const
   {
diff --git a/sql/item_sum.h b/sql/item_sum.h
index e221d04397d..77d1809d7bf 100644
--- a/sql/item_sum.h
+++ b/sql/item_sum.h
@@ -355,7 +355,8 @@ public:
     ROW_NUMBER_FUNC, RANK_FUNC, DENSE_RANK_FUNC, PERCENT_RANK_FUNC,
     CUME_DIST_FUNC, NTILE_FUNC, FIRST_VALUE_FUNC, LAST_VALUE_FUNC,
     NTH_VALUE_FUNC, LEAD_FUNC, LAG_FUNC, PERCENTILE_CONT_FUNC,
-    PERCENTILE_DISC_FUNC, SP_AGGREGATE_FUNC, JSON_ARRAYAGG_FUNC
+    PERCENTILE_DISC_FUNC, SP_AGGREGATE_FUNC, JSON_ARRAYAGG_FUNC,
+    JSON_OBJECTAGG_FUNC
   };
 
   Item **ref_by; /* pointer to a ref to the object used to register it */
diff --git a/sql/sql_window.cc b/sql/sql_window.cc
index a9a1dccf9f6..9e6ef6468e0 100644
--- a/sql/sql_window.cc
+++ b/sql/sql_window.cc
@@ -2936,6 +2936,14 @@ bool Window_func_runner::add_function_to_run(Item_window_func *win_func)
       my_error(ER_NOT_SUPPORTED_YET, MYF(0),
                "COUNT(DISTINCT) aggregate as window function");
       return true;
+    case Item_sum::JSON_ARRAYAGG_FUNC:
+      my_error(ER_NOT_SUPPORTED_YET, MYF(0),
+               "JSON_ARRAYAGG() aggregate as window function");
+      return true;
+    case Item_sum::JSON_OBJECTAGG_FUNC:
+      my_error(ER_NOT_SUPPORTED_YET, MYF(0),
+               "JSON_OBJECTAGG() aggregate as window function");
+      return true;
     default:
       break;
   }