NullPointerException at 'itemSubjectRef' parsing of BPMN file

2017-03-27 16:24:25,613 [main] ERROR (null: 25, 102): cvc-attribute.3: The value 'broken_reference.bpmn#test' of attribute 'itemSubjectRef' on element 'dataOutput' is not valid with respect to its type, 'QName'.
java.lang.NullPointerException
	at org.jbpm.bpmn2.xml.TaskHandler.readIoSpecification(TaskHandler.java:145)
	at org.jbpm.bpmn2.xml.TaskHandler.handleNode(TaskHandler.java:79)
	at org.jbpm.bpmn2.xml.UserTaskHandler.handleNode(UserTaskHandler.java:47)
	at org.jbpm.bpmn2.xml.TaskHandler.end(TaskHandler.java:267)
	at org.drools.core.xml.ExtensibleXmlParser.endElement(ExtensibleXmlParser.java:439)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(AbstractSAXParser.java:609)
	at com.sun.org.apache.xerces.internal.impl.xs.XMLSchemaValidator.endElement(XMLSchemaValidator.java:863)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(XMLDocumentFragmentScannerImpl.java:1782)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2973)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:606)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:117)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:510)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:848)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:777)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1213)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:649)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl.parse(SAXParserImpl.java:333)
	at org.drools.core.xml.ExtensibleXmlParser.read(ExtensibleXmlParser.java:318)
	at org.drools.core.xml.ExtensibleXmlParser.read(ExtensibleXmlParser.java:180)
	at org.jbpm.compiler.xml.XmlProcessReader.read(XmlProcessReader.java:61)
	at org.jbpm.bpmn2.xml.XmlBPMNProcessDumper.readProcess(XmlBPMNProcessDumper.java:914)