problem with XercesparerLiaison

Hi, i have a problem with XercesparerLiaison
if i use it, for parse a xml document the xpath expresion than i evaluate don´t work, the error is: "prefix must resolver to a namespace"
but if i use XalanSourceTreeParserLiaison with he same document and the same xpath expresion the evaluation works fine...,
i need use XercesParserLiaison for use the method mapToXercesDocument,