Why is an expression of this sort "Obj x = new Obj();" causing compilation error, if B is subclass of A?

Question

I am trying to run this code:

class A {}
class B extends A {}
class Obj<T> {
    private T c;
    public void set(T a) {
        c=a;
    }
}
public class Main {
    public static void main(String[] args) {
        Obj<A> a = new Obj<B>();
    }
}

And I am getting the following Exception:
Exception in thread "main" java.lang.Error: Unresolved compilation problem: Type mismatch: cannot convert from Obj<B> to Obj<A>
Should't it be able to work if B is a subclass of A?