I created a new datatype named Text but set the wrong type in UML. The result was:
<property name="xml" type="$attribute.type.fullyQualifiedHibernateType">
<column name="XML" not-null="true" unique="false" sql-type="TEXT"/>
in my .hbm.xml file.
According to Chad, an OCL constraint that checks the type should be added.
This was spotted using the Spring cartridge so perhaps this needs to be added to different subprojects?