API isUserInGroup method doesn't work with sub group

Hello,

If I have a main group with some sub group: ie group1 and group2.
If my user is in group1, $MyUser.isUserInGroup("XWiki.MainGroup") return false instead true.
(Method working well if I fill MainGroup with users)

Here a piece of code to test

{{velocity}}
#set($Grp="XWiki.MyGroup")
#set($user=$xwiki.getUser())
$user.isUserInGroup($Grp)
{{/velocity}}

Thxs