[url] tags break: domains with non ASCII characters like an umlaut

I'm currently working on a website running on a domain containing an umlaut (ä, ü, ï for example). When I use the following BB code, it's not being parsed, but the code is simply displayed on the page: [url]http://exämple.com/[/url] (Obfuscated domain name for privacy reasons). As soon as I remove the character containing the umlaut, the link works like a charm.