diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index 4eeeeaf0a6..058aade2af 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -1111,7 +1111,14 @@ json_object_agg_transfn_worker(FunctionCallInfo fcinfo,
 
 	if (unique_keys)
 	{
-		const char *key = &out->data[key_offset];
+		/*
+		 * Copy the key first, instead of pointing into the buffer. It will be
+		 * added to the hash table, but the buffer may get reallocated as
+		 * we're appending more data to it. That would invalidate pointers to
+		 * keys in the current buffer.
+		 */
+		const char *key = MemoryContextStrdup(aggcontext,
+											  &out->data[key_offset]);
 
 		if (!json_unique_check_key(&state->unique_check.check, key, 0))
 			ereport(ERROR,
@@ -1274,8 +1281,15 @@ json_build_object_worker(int nargs, const Datum *args, const bool *nulls, const
 
 		if (unique_keys)
 		{
-			/* check key uniqueness after key appending */
-			const char *key = &out->data[key_offset];
+			/*
+			 * check key uniqueness after key appending
+			 *
+			 * Copy the key first, instead of pointing into the buffer. It
+			 * will be added to the hash table, but the buffer may get
+			 * reallocated as we're appending more data to it. That would
+			 * invalidate pointers to keys in the current buffer.
+			 */
+			const char *key = pstrdup(&out->data[key_offset]);
 
 			if (!json_unique_check_key(&unique_check.check, key, 0))
 				ereport(ERROR,
diff --git a/src/test/regress/expected/json.out b/src/test/regress/expected/json.out
index 7df11c2f38..96c40911cb 100644
--- a/src/test/regress/expected/json.out
+++ b/src/test/regress/expected/json.out
@@ -2330,6 +2330,9 @@ select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');
  {"a" : "1", "b" : "2", "" : "3", "d e f" : "a b c"}
 (1 row)
 
+-- json_object_agg_unique requires unique keys
+select json_object_agg_unique(mod(i,100), i) from generate_series(0, 199) i;
+ERROR:  duplicate JSON object key value: "0"
 -- json_to_record and json_to_recordset
 select * from json_to_record('{"a":1,"b":"foo","c":"bar"}')
     as x(a int, b text, d text);
diff --git a/src/test/regress/expected/sqljson.out b/src/test/regress/expected/sqljson.out
index 4f91e2117e..9adfbe15f6 100644
--- a/src/test/regress/expected/sqljson.out
+++ b/src/test/regress/expected/sqljson.out
@@ -537,6 +537,8 @@ SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2 ABSENT ON NULL);
  {"a" : "1", "c" : 2}
 (1 row)
 
+SELECT JSON_OBJECT(1: 1, '2': NULL, '3': 1, repeat('x', 1000): 1, 2: repeat('a', 100) WITH UNIQUE);
+ERROR:  duplicate JSON object key value: "2"
 SELECT JSON_OBJECT(1: 1, '1': NULL WITH UNIQUE);
 ERROR:  duplicate JSON object key value: "1"
 SELECT JSON_OBJECT(1: 1, '1': NULL ABSENT ON NULL WITH UNIQUE);
@@ -921,6 +923,9 @@ FROM (VALUES (1, 1), (0, NULL),(4, null), (5, null),(6, null),(2, 2)) foo(k, v);
  {"1": 1, "2": 2}
 (1 row)
 
+SELECT JSON_OBJECTAGG(mod(i,100): (i)::text FORMAT JSON WITH UNIQUE)
+FROM generate_series(0, 199) i;
+ERROR:  duplicate JSON object key value: "0"
 -- Test JSON_OBJECT deparsing
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT JSON_OBJECT('foo' : '1' FORMAT JSON, 'bar' : 'baz' RETURNING json);
diff --git a/src/test/regress/sql/json.sql b/src/test/regress/sql/json.sql
index 5c886cd6b3..8251f4f400 100644
--- a/src/test/regress/sql/json.sql
+++ b/src/test/regress/sql/json.sql
@@ -755,6 +755,8 @@ select json_object('{a,b,NULL,"d e f"}','{1,2,3,"a b c"}');
 
 select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');
 
+-- json_object_agg_unique requires unique keys
+select json_object_agg_unique(mod(i,100), i) from generate_series(0, 199) i;
 
 -- json_to_record and json_to_recordset
 
diff --git a/src/test/regress/sql/sqljson.sql b/src/test/regress/sql/sqljson.sql
index bb2487e864..42dbec26d6 100644
--- a/src/test/regress/sql/sqljson.sql
+++ b/src/test/regress/sql/sqljson.sql
@@ -138,6 +138,8 @@ SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2);
 SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2 NULL ON NULL);
 SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2 ABSENT ON NULL);
 
+SELECT JSON_OBJECT(1: 1, '2': NULL, '3': 1, repeat('x', 1000): 1, 2: repeat('a', 100) WITH UNIQUE);
+
 SELECT JSON_OBJECT(1: 1, '1': NULL WITH UNIQUE);
 SELECT JSON_OBJECT(1: 1, '1': NULL ABSENT ON NULL WITH UNIQUE);
 SELECT JSON_OBJECT(1: 1, '1': NULL NULL ON NULL WITH UNIQUE RETURNING jsonb);
@@ -283,6 +285,9 @@ FROM (VALUES (1, 1), (1, NULL), (2, 2)) foo(k, v);
 SELECT JSON_OBJECTAGG(k: v ABSENT ON NULL WITH UNIQUE KEYS RETURNING jsonb)
 FROM (VALUES (1, 1), (0, NULL),(4, null), (5, null),(6, null),(2, 2)) foo(k, v);
 
+SELECT JSON_OBJECTAGG(mod(i,100): (i)::text FORMAT JSON WITH UNIQUE)
+FROM generate_series(0, 199) i;
+
 -- Test JSON_OBJECT deparsing
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT JSON_OBJECT('foo' : '1' FORMAT JSON, 'bar' : 'baz' RETURNING json);