Hibernate + EhCache = java.io.NotSerializableException

I'm doing load test now. After approximate 900 requests i got this error:

[ 03.08.10 11:49:00.465] [Store org.hibernate.cache.StandardQueryCache Spool Thread] ERROR net.sf.ehcache.store.DiskStore  - org.hibernate.cache.StandardQueryCacheCache: Failed to write element to disk 'sql: select hotelfeatu0_.name as col_0_0_ from feature hotelfeatu0_ where hotelfeatu0_.class='org.company.domains.HotelFeatures' and hotelfeatu0_.lang=? and hotelfeatu0_.hotel_id=?; parameters: ; named parameters: {ht=100: 'Hotel', lng=en}'. Initial cause was org.company.domains.Hotel
java.io.NotSerializableException: org.company.domains.Hotel
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
    at java.util.HashMap.writeObject(HashMap.java:1001)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:945)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1461)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
    at java.io.ObjectOutputStream.writeOrd