public class InternalStateEntity<I> {
    private class B {
        public B() { }
    }
    private List<B> b = new ArrayList<>();
    protected InternalStateEntity() {
        b.add(new B());
    }
    public List<B> getB() {
        return b;
    }
}

public class MongoStateEntity<K> extends InternalStateEntity<ObjectId> {
    public MongoStateEntity() { }
}

Exception in thread "main" org.bson.codecs.configuration.CodecConfigurationException: An exception occurred when encoding using the AutomaticPojoCodec.
Encoding a MongoStateEntity: 'pojo.MongoStateEntity@37883b97' failed with the following exception:

MongoStateEntity contains generic types that have not been specialised.
Top level classes with generic types are not supported by the PojoCodec.

A custom Codec or PojoCodec may need to be explicitly configured and registered to handle this type.
        at org.bson.codecs.pojo.AutomaticPojoCodec.encode(AutomaticPojoCodec.java:53)
        at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:63)
        at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:29)
        at com.mongodb.internal.connection.SplittablePayload$WriteRequestEncoder.encode(SplittablePayload.java:200)
        at com.mongodb.internal.connection.SplittablePayload$WriteRequestEncoder.encode(SplittablePayload.java:187)
        at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:63)
        at org.bson.codecs.BsonDocumentWrapperCodec.encode(BsonDocumentWrapperCodec.java:29)
        at com.mongodb.internal.connection.BsonWriterHelper.writeDocument(BsonWriterHelper.java:77)
        at com.mongodb.internal.connection.BsonWriterHelper.writePayload(BsonWriterHelper.java:59)
        at com.mongodb.internal.connection.CommandMessage.encodeMessageBodyWithMetadata(CommandMessage.java:145)
...

TypeParameterMap{fieldToClassParamIndexMap={}}

TypeParameterMap{fieldToClassParamIndexMap={0=Either{left=null, right=TypeParameterMap{fieldToClassParamIndexMap={}}}}}