JUnit test for a constructor casting a parent to child

I am writing test cases for Class A given below. but facing class cast exception bcoz of constructor casting a class into another.

class A {
  private B b;

  @Autowired
  private RestApi api;

  public A(D d){
     super(d);
     this.b = (B)d; 
  }

  public someMethod(){
      // Logic
  }
}

where D is a parent class and Class B extends Class D.

In the Constructor I am casting Parent to child.

How to handle This constructor casting into Mockito junit.