Generics and casting - cannot cast inherited class to base class

I know this is old, yet I am still not very good with understanding those problems. Can anyone tell me why the following does not work (throws a runtime exception about casting)?

public abstract class EntityBase { }
public class MyEntity : EntityBase { }

public abstract class RepositoryBase<T> where T : EntityBase { }
public class MyEntityRepository : RepositoryBase<MyEntity> { }

And now the casting line:

MyEntityRepository myEntityRepo = GetMyEntityRepo(); // whatever
RepositoryBase<EntityBase> baseRepo = (RepositoryBase<EntityBase>)myEntityRepo;

So, can anyone explain how is this invalid? And, I you are not in the mood to explain - is there a line of code I can use to actually do this cast?