Problem, unable to commit a clob update transaction

Insert(persist) of entity with clob work fine

JBoss version: 4.0.3RC2
mySQL version 4.1.12a

Database table description:

...
cDescription longtext
...

Entity code:

@Entity
@Table(name = "COMPETENCE")
public class Competence implements java.io.Serializable {
 ...
 private String description;
 ...

 @Column(name="cDescription", nullable=false)
 @Lob(type=LobType.CLOB,fetch=FetchType.LAZY)
 public String getDescription() {
 return this.description;
 }

 public void setDescription(String description) {
 this.description = description;
 }
}

When I try to update(merge) entity with clob, I got "Unable to Commit" exception...

2005-09-21 15:50:03,125 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[localhost].[/internal].[action]] Servlet.service() for servlet action threw exception
java.lang.RuntimeException: org.jboss.tm.JBossRollbackException: Unable to commit, tx=TransactionImpl:XidImpl[FormatId=257, GlobalId=audun/10, BranchQual=, localId=10] status=STATUS_NO_TRANSACTION; - nested throwable: (org.hibernate.PropertyValueException: not-null property references a null or transient value: no.antares.domain.Competence.description)
 at org.jboss.aspects.tx.TxPolicy.handleEndTransactionException(TxPolicy.java:185)

Is it a bug since it work with persist but not with merge, or ?