title: JPA QueryException - A reference class must be provided
body: <p>Using JPA 2.0, Java EE 5, Weblogic 10.3 (11g), JDK 6, EclipseLink.</p>
  
  <p>When i attempt to run this:</p>
  
  <pre><code> CriteriaQuery _criteriaquery = EM.getCriteriaBuilder().createQuery(Clazz);
          Query query = EM.createQuery(_criteriaquery);
          return query.getResultList();
  </code></pre>
  
  <p>I just get this :</p>
  
  <pre><code>Caused by: Exception [EclipseLink-6029] (Eclipse Persistence Services - 2.1.2.v20101206-r8635): org.eclipse.persistence.exceptions.QueryException
      Exception Description: A reference class must be provided.
      Query: ReportQuery()
          at org.eclipse.persistence.exceptions.QueryException.referenceClassMissing(QueryException.java:1004)
          at org.eclipse.persistence.queries.ObjectLevelReadQuery.checkDescriptor(ObjectLevelReadQuery.java:744)
          at org.eclipse.persistence.queries.DatabaseQuery.execute(DatabaseQuery.java:661)
  </code></pre>
  
  <p>What could be the cause of this error ?</p>