I am trying to evaluate a XPathExpression with XQSharp and get the Error:"Type check error. The empty sequence cannot be cast to type 'xs:integer'."

AltovaXMLSpy evaluates it correct as "true".

oXmlDoc.CreateNavigator().XPathEvaluate("root/foo/bar cast as xs:integer lt count(root/blah/blub)", oNamespaseManager).ToString()

This XML looks like this:

<root xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <foo>
        <bar>0</bar>
    </foo>
    <blah>
        <blub/> 
    </blah>
</root>

What am I missing?