Error with persisting Entity

[xybrek via StackOverflow]

I'm encountering the error below when trying to persist a Entity with my Appengine web app.

Encountered a problem: ormLibs cannot be null

Please see the logs [C:\Users\XYBREK~1\AppData\Local\Temp\enhance1265959303797845825.log] for further information.

Error log:

java.lang.NullPointerException: ormLibs cannot be null at com.google.appengine.tools.enhancer.EnhancerLoader.removeOrmLibs(EnhancerLoader.java:66) at com.google.appengine.tools.enhancer.EnhancerLoader.getClassPath(EnhancerLoader.java:43) at com.google.appengine.tools.enhancer.EnhancerLoader.(EnhancerLoader.java:37) at com.google.appengine.tools.enhancer.Enhancer.execute(Enhancer.java:69) at com.google.appengine.tools.enhancer.Enhance.(Enhance.java:71) at com.google.appengine.tools.enhancer.Enhance.main(Enhance.java:51)

This is the Entity to be persisted:

@SuppressWarnings("serial")
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Account implements Serializable {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Long id;
    @Persistent
    private String username;
    @Persistent 
    private String password;

    public Account() {

    }

    // Code omitted

}

What could be the reason for this error?

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http://stackoverflow.com/questions/11170144/error-with-persisting-entity