Extended HashMap as Embeddable class

Hi,

Should it be possible to create an Embeddable class that extends HashMap (or Set etc.)? I can user HashMaps  as parameter type in entities, but when I extend it and annotate it with @Embeddable annotation - I get an error.

@Embeddable
public class A extends HashMap<String, String> {
(...)
}

User defined persistable classes that extend Java collection classes are not supported.