Prepared statement fails using JSON_OBJECT, UTF-8 value and null value

Executing a prepared statement using JSON_OBJECT(), a UTF-8 value and a null value fails.

CREATE TABLE bug_json_utf8 (
id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
var text COLLATE utf8mb4_unicode_ci);
PREPARE stmt FROM "INSERT INTO bug_json_utf8 SET id = NULL, var = JSON_OBJECT('text1',?,'text2',?)";

1. This fails with Incorrect string value: '\xE9", "t...' for column 'var' at row 1
   EXECUTE stmt USING 'café',null;

1. These work :
   EXECUTE stmt USING 'café','café';
   EXECUTE stmt USING null,null;

DEALLOCATE PREPARE stmt;