diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index f704418603..192f9abae1 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -2003,7 +2003,7 @@ json_object_agg_transfn(PG_FUNCTION_ARGS)
 		if (arg_type == InvalidOid)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("could not determine data type for argument 1")));
+					 errmsg("could not determine data type for argument %d", 1)));
 
 		json_categorize_type(arg_type, &state->key_category,
 							 &state->key_output_func);
@@ -2013,7 +2013,7 @@ json_object_agg_transfn(PG_FUNCTION_ARGS)
 		if (arg_type == InvalidOid)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("could not determine data type for argument 2")));
+					 errmsg("could not determine data type for argument %d", 2)));
 
 		json_categorize_type(arg_type, &state->val_category,
 							 &state->val_output_func);
-- 
2.11.0