Picketlink: XMLStreamException: Can not create a STaX writer for a DOMResult -- not (yet?) implemented.

Issue

I'm getting the following exception from Picketlink when it tries to authenticate a user:

Caused by: org.picketlink.identity.federation.core.exceptions.ProcessingException: PLFED000102: Processing Exception
    at org.picketlink.identity.federation.PicketLinkLoggerImpl.processingError(PicketLinkLoggerImpl.java:85) [picketlink-jbas7-2.1.9.SP2-redhat-1.jar:2.1.9.SP2-redhat-1]
    at org.picketlink.identity.federation.core.util.StaxUtil.getXMLStreamWriter(StaxUtil.java:128) [picketlink-core-2.1.9.SP2-redhat-1.jar:2.1.9.SP2-redhat-1]
    at org.picketlink.identity.federation.core.wstrust.writers.WSTrustRequestWriter.<init>(WSTrustRequestWriter.java:100) [picketlink-core-2.1.9.SP2-redhat-1.jar:2.1.9.SP2-redhat-1]
    at org.picketlink.identity.federation.core.wstrust.STSClient.createSourceFromRequest(STSClient.java:445) [picketlink-core-2.1.9.SP2-redhat-1.jar:2.1.9.SP2-redhat-1]
    ... 35 more
Caused by: javax.xml.stream.XMLStreamException: Can not create a STaX writer for a DOMResult -- not (yet?) implemented.
    at com.ctc.wstx.stax.WstxOutputFactory.createSW(WstxOutputFactory.java:314) [wstx-asl-3.2.1.jar:3.2.1]
    at com.ctc.wstx.stax.WstxOutputFactory.createXMLStreamWriter(WstxOutputFactory.java:129) [wstx-asl-3.2.1.jar:3.2.1]
    at __redirected.__XMLOutputFactory.createXMLStreamWriter(__XMLOutputFactory.java:137) [jboss-modules.jar:1.3.0.Final-redhat-2]
    at org.picketlink.identity.federation.core.util.StaxUtil.getXMLStreamWriter(StaxUtil.java:126) [picketlink-core-2.1.9.SP2-redhat-1.jar:2.1.9.SP2-redhat-1]
    ... 37 more