diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index ef5608deb82..aa197f74142 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -85,6 +85,7 @@ static void json_categorize_type(Oid typoid,
 					 Oid *outfuncoid);
 static void datum_to_json(Datum val, bool is_null, StringInfo result,
 			  JsonTypeCategory tcategory, Oid outfuncoid);
+static text *catenate_stringinfo_string(StringInfo buffer, const char *addon);
 
 /* the null action object used for pure validation */
 static JsonSemAction nullSemAction =
@@ -166,7 +167,7 @@ lex_expect(JsonParseContext ctx, JsonLexContext *lex, JsonTokenType token)
 
 /* utility function to check if a string is a valid JSON number */
 extern bool
-IsValidJsonNumber(const char * str, int len)
+IsValidJsonNumber(const char *str, int len)
 {
 	bool		numeric_error;
 	JsonLexContext dummy_lex;
@@ -191,7 +192,7 @@ IsValidJsonNumber(const char * str, int len)
 
 	json_lex_number(&dummy_lex, dummy_lex.input, &numeric_error);
 
-	return ! numeric_error;
+	return !numeric_error;
 }
 
 /*
@@ -1359,6 +1360,7 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 			break;
 		case JSONTYPE_NUMERIC:
 			outputstr = OidOutputFunctionCall(outfuncoid, val);
+
 			/*
 			 * Don't call escape_json for a non-key if it's a valid JSON
 			 * number.
@@ -1646,6 +1648,8 @@ to_json(PG_FUNCTION_ARGS)
 
 /*
  * json_agg transition function
+ *
+ * aggregate input column as a json array value.
  */
 Datum
 json_agg_transfn(PG_FUNCTION_ARGS)
@@ -1732,12 +1736,32 @@ json_agg_finalfn(PG_FUNCTION_ARGS)
 
 	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
 
+	/* NULL result for no rows in, as is standard with aggregates */
 	if (state == NULL)
 		PG_RETURN_NULL();
 
-	appendStringInfoChar(state, ']');
+	/* Else return state with appropriate array terminator added */
+	PG_RETURN_TEXT_P(catenate_stringinfo_string(state, "]"));
+}
 
-	PG_RETURN_TEXT_P(cstring_to_text(state->data));
+/*
+ * Helper function for aggregates: return given StringInfo's contents plus
+ * specified trailing string, as a text datum.  We need this because aggregate
+ * final functions are not allowed to modify the aggregate state.
+ */
+static text *
+catenate_stringinfo_string(StringInfo buffer, const char *addon)
+{
+	/* custom version of cstring_to_text_with_len */
+	int			buflen = buffer->len;
+	int			addlen = strlen(addon);
+	text	   *result = (text *) palloc(buflen + addlen + VARHDRSZ);
+
+	SET_VARSIZE(result, buflen + addlen + VARHDRSZ);
+	memcpy(VARDATA(result), buffer->data, buflen);
+	memcpy(VARDATA(result) + buflen, addon, addlen);
+
+	return result;
 }
 
 /*