problem with UTF-8

hello
i have just installed xindice v1.1b4, i have created an empty collection, but when i try to add xml files to it i get this error:
ERROR : Invalid byte 2 of 3-byte UTF-8 sequence.
java.io.UTFDataFormatException: Invalid byte 2 of 3-byte UTF-8 sequence.
        at org.apache.xerces.impl.io.UTF8Reader.invalidByte(Unknown Source)
        at org.apache.xerces.impl.io.UTF8Reader.read(Unknown Source)
        at org.apache.xerces.impl.XMLEntityScanner.load(Unknown Source)
        at org.apache.xerces.impl.XMLEntityScanner.scanContent(Unknown Source)
        at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanContent(Unknown Source)
        at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
        at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
        at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
        at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
        at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
        at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
        at org.apache.xindice.tools.command.AddDocument.execute(AddDocument.java:81)
        at org.apache.xindice.tools.XMLTools.execute(XMLTools.java:293)
        at org.apache.xindice.tools.XMLTools.process(XMLTools.java:171)
        at org.apache.xindice.tools.XMLTools.main(XMLTools.java:82)

It seems that my file is not UTF-8 valid, but if i open it, it seems all ok, what can i do?
thank you