The join command takes an array as it's first argument: therefore you must use square brackets [ ] or an array. Try
"[_x] join ......
The lesson is, always[/i] check the syntax in the command ref. The vast majority of answers in the forum are fundamentally correct (like BibiPhoque's here): however, many (especially mine) contain minor errors of syntax. It is not the duty of the answerer to check these details: it is the duty of the questioner.
In any case you shouldn't use a command unless you understand it. If you have read the command ref (both online version and unofficial version) and don't understand a command, by all means ask here.