Mockito : Stubbing some methods of spy objects

I'm using Mockito for unit testing a class methods. However, If my test method uses some Helper/Utils class methods then I'm spying on that Helper/Utils class objects.In some cases I also need to stub the spy object methods for checking the exceptions. I use

@Spy
private itemsDetailsHelper

doReturn(null).when(itemsDetailsHelper).getDistinctItemsList(itemDetails);

But the method is not returning null.