Thread:Project:Support desk/Problem using Extension:Interwiki with MW 1.18

When I try to add or edit prefixes, it gives an error.

Fatal error: Call to undefined method Xml::hidden in /home/content/01/7537801/html/EN-40K Sturmkrieg/extensions/Interwiki/Interwiki_body.php on line 165

I'm using the 1.17 version, which on the extension page is the same for 1.18.