Why do I see a "Adding multiple last resources is disallowed" warning in JBoss when I only have a single DataSource?

Issue

I only have one datasource (DefaultDS) for one database, and my app uses DefaultDS as its datasource. I'm using a local-tx-datasource for the DefaultDS, but it seems that should be all that is necessary, seeing as I'm using the one database for everything (and I've disabled hypersonic).

WARN  [com.arjuna.ats.internal.jta.transaction.arjunacore.lastResource.disallow]
Adding multiple last resources is disallowed. Current resource is
org.jboss.resource.connectionmanager.TxConnectionManager$LocalXAResource@10a9f69