Mockito - mock a method call in constructor

I have a class X which takes in a YBuilder and constructs an instance of Y

public class X {
  private Y y;
  public X(YBuilder builder) {
    y = builder.build();
  }
}

I need to mock all calls to Y, so I have written my Unit tests like this:

@Mock
private Y Y;

@Mock
private YBuilder builder;

@InjectMocks
private X x;

@Before
public void setup() {
  when(builder.build()).thenReturn(y); // this does not work
}

I get a Null Pointer Exception in my main class since the dependency Y has not been mocked.

I think this is due to the fact that @InjectMocks instantiates a new instance of X before I am able to mock the call.

How can I fix this? Any inputs are appreciated.