Aliases redirect to wp-signup.php?new=alias.com

Hi,

When I go on mysite.com (multisite), I go to the homepage (normal behavior).
But when I set an alias, there's a problem. Let's say alias.com is an alias of mysite.com.
If I go to alias.com, I go to http://mysite.com/wp-signup.php?new=alias.com

Any idea?
Best Regards

PS: I think I need to answer this question in order to get Domain mapping to work.