How to store a property that is subclass of class in Mongo DB document using Morphia?

For Example :

I have a class

abstract class A implements Serializable{

 //some fields

}

class B extends A {
//fields specific to this class
}

class C extends A {
//fields specific to this class
}

class MyData Serializable {
 String someProperty;
 B instanceOfB; 
 C instanceOfC;

 // I need to use B,C instead of A 
}

When I store MyData using morphia , I get the error : "IllegalArgumentException: Cannot serialize class B"