StreamCorruptedException: invalid stream header mapping result set when using EJB2.1 entity beans on JBoss

Issue

We're upgrading JBOSS 4.3 CP10 to 5.1.1 and we're encountering an issue loading a BigDecimal field on an Entity Bean (EJB2.1).

We're getting a StremCorruptedException in JDBCUtil.converToObject

Caused by: java.sql.SQLException: Unable to load to deserialize result: java.io.StreamCorruptedException: invalid stream header: 020401EF
        at org.jboss.ejb.plugins.cmp.jdbc.JDBCUtil.convertToObject(JDBCUtil.java:305)
        at org.jboss.ejb.plugins.cmp.jdbc.JDBCUtil.convertToObject(JDBCUtil.java:254)
        at org.jboss.ejb.plugins.cmp.jdbc.JDBCResultSetReader$4.readResult(JDBCResultSetReader.java:153)
        at org.jboss.ejb.plugins.cmp.jdbc.JDBCResultSetReader$AbstractResultSetReader.get(JDBCResultSetReader.java:488)
        at org.jboss.ejb.plugins.cmp.jdbc.bridge.JDBCAbstractCMPFieldBridge.loadArgumentResults(JDBCAbstractCMPFieldBridge.java:482)

This exception happens when processing a DECIMAL db field into a java.math.BigDecimal.

Our standardjbosscmp-jdbc.xml for BigDecimal is:

         <mapping>
            <java-type>java.math.BigDecimal</java-type>
            <jdbc-type>DECIMAL</jdbc-type>
            <sql-type>DECIMAL</sql-type>
         </mapping>

I also tried no mapping and

           <mapping>
            <java-type>java.math.BigDecimal</java-type>
            <jdbc-type>NUMERIC</jdbc-type>
            <sql-type>DECIMAL</sql-type>
         </mapping>

regardless of the mapping I get the same error.

Our database is MSSQL 2008 R2, with MSSQL JDBC Driver, db column is DECIMAL(7,2). We're running vanilla 5.1.1 on Windows 7 64 bit with JDK 1.6.0_27