Error while specifying keyref in xml

[revenge]

This is my code:\[code\] <xs:key name="UserIdKey"> <xs:selector xpath="Users/User"></xs:selector> <xs:field xpath="@Id"></xs:field> </xs:key> <xs:keyref name="FriendsIdKeyRef" refer="UserIdKey"> <xs:selector xpath="Friends/User"></xs:selector> <xs:field xpath="@FriendId"></xs:field> </xs:keyref>\[/code\]I get the following error:\[quote\] Attribute refer='UserIdKey' does not refer to a key or unique constraint.\[/quote\]But the \[code\]UserIdKey\[/code\] is right there. What am I doing wrong?Thanks.