MongoDB spring repository - abstract class as field "Class is abstract"

I'm getting error when spring mongo template reading object from DB: "Class is abstract". This is because internal field in document is abstract type. in my case classes looks like:

public abstract class Context {
    private String name;
}

public class AContext {
    private String aData;
}

public class BContext {
    private String bData;
}

@Document
@TypeAlias("Task")
public class Task {
    @Id
    private String id;
    private String name;
    private List<Context> contexts;
}

How can I fix thiss issue ?