The API reject some document titles (in my case, one containing 🛫, when using mysql as the database backend and its default 'utf8' field type. This one rejects UTF-8 characters longer than 3 bytes, see https://stackoverflow.com/questions/20411440/incorrect-string-value-xf0-x9f-x8e-xb6-xf0-x9f-mysql , https://stackoverflow.com/questions/30074492/what-is-the-difference-between-utf8mb4-and-utf8-charsets-in-mysql and https://dev.mysql.com/doc/refman/8.0/en/charset-unicode-sets.html.