Mock itself throwing exception

I am trying to mock a function to throw an exception like this

BigQueryIO mockb = mock(BigQueryIO.class);

when(mockb.getClass().getDeclaredField("SOME_FIELD"))
        .thenThrow(new NoSuchFieldException());

it's throwing NoSuchFieldException in the test mock itself.

I have also tried this way

when(BigQueryIO.class.getDeclaredField("SOME_FIELD"))
        .thenThrow(new NoSuchFieldException());

but this is also not working,

Am I missing some basic thing? Does mock work differently with .class?