Let's consider following XSD and XML document:

    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

    <xsd:element name="root">
       <xsd:complexType>
          <xsd:sequence>
             <xsd:element name="str" type="xsd:string" fixed="abc"/>
             <xsd:element name="number" type="xsd:integer" fixed="123"/>
             <xsd:element name="bool" type="xsd:boolean" fixed="true"/>
          </xsd:sequence>
       </xsd:complexType>
    </xsd:element>
    </xsd:schema>


    <root>
       <str/>
       <number/>
       <bool/>
    </root>

when this XML document is unmarshlled following error occurs:

    javax.xml.bind.UnmarshalException: Zero length BigInteger
     - with linked exception:
    [java.lang.NumberFormatException: Zero length BigInteger]
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.handleEvent(UnmarshallingContext.java:728)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.handleError(UnmarshallingContext.java:755)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.handleError(UnmarshallingContext.java:751)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.Loader.handleParseConversionException(Loader.java:260)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.LeafPropertyLoader.text(LeafPropertyLoader.java:54)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.text(UnmarshallingContext.java:574)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.ValidatingUnmarshaller.text(ValidatingUnmarshaller.java:115)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.processText(SAXConnector.java:198)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.endElement(SAXConnector.java:161)
            at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(AbstractSAXParser.java:608)
            at com.sun.org.apache.xerces.internal.parsers.AbstractXMLDocumentParser.emptyElement(AbstractXMLDocumentParser.java:183)
            at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.scanStartElement(XMLNSDocumentScannerImpl.java:355)
            at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2790)
            at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:605)
            at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:116)
            at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:511)
            at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:876)
            at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:805)
            at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:140)
            at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1212)
            at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:648)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:243)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:214)
            at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:157)
            at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:162)
            at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:171)
            at Test7.test(Test7.java:214)
            at Test7.main(Test7.java:109)
    Caused by: java.lang.NumberFormatException: Zero length BigInteger
            at java.math.BigInteger.<init>(BigInteger.java:397)
            at java.math.BigInteger.<init>(BigInteger.java:583)
            at com.sun.xml.internal.bind.DatatypeConverterImpl._parseInteger(DatatypeConverterImpl.java:76)
            at com.sun.xml.internal.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$21.parse(RuntimeBuiltinLeafInfoImpl.java:777)
            at com.sun.xml.internal.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$21.parse(RuntimeBuiltinLeafInfoImpl.java:775)
            at com.sun.xml.internal.bind.v2.runtime.reflect.TransducedAccessor$CompositeTransducedAccessorImpl.parse(TransducedAccessor.java:230)
            at com.sun.xml.internal.bind.v2.runtime.unmarshaller.LeafPropertyLoader.text(LeafPropertyLoader.java:50)
            ... 23 more

But it seems that this documents should be processed correctly as per W3C spec [1]:

    If fixed is specified, then the element's content must either be empty, in which case fixed behaves as default, or its value must match the supplied constraint value.

The minimized test case is attached, actually there are two XML documents causing this error.

In order to reproduce this error:

1. Extract attached archive to some directory on Windows platform, say A.
2. Change current directory to directory A.
3. Run test7-run2.bat.

The generated classes along with some other stuff generated from minimized testcase are attached as well.

[1] http://www.w3.org/TR/xmlschema-1/#Element_Declaration_details