When upgrading to 6.1.x from 6.0.x it is not possible anymore to serialize entities with Ref fields. The class com.googlecode.objectify.Ref does not implement the Serializable interface anymore. What's the reason for it and how to solve the problem?

org.apache.commons.lang3.SerializationException: java.io.NotSerializableException: com.googlecode.objectify.Ref at org.apache.commons.lang3.SerializationUtils.serialize(SerializationUtils.java:266) at org.apache.commons.lang3.SerializationUtils.serialize(SerializationUtils.java:241) at org.apache.commons.lang3.SerializationUtils.clone(SerializationUtils.java:139)