Cannot alias primitive types

I was unable to alias primitive types.
I tried the following:
XStream xs = new XStream( );
xs.alias( "bo", Boolean.TYPE );
Object o = new boolean[]

;
String xml = xs.toXML( o );

that results in

<bo-array>
<bo>true</bo>
<bo>false</bo>
</bo-array>

but the call to xs.fromXML() for this XML throws
com.thoughtworks.xstream.mapper.CannotResolveClassException: boolean : boolean