Mocked private method is called

I'm using PowerMockito for mocking private method. But instead being mocked it is called. I need to test strangeMethod(), which calls privateMethod().

Here is my class for testing:

public class ExampleService {
public String strangeMethod() {
        privateMethod();
        return "Done!";
    }

    private String privateMethod() {
        throw new UnsupportedOperationException();
    }
}

My test method:

@Test
    public void strangeMethodTest() throws Exception {
        ExampleService exampleService = PowerMockito.spy(new ExampleService());
        PowerMockito.when(exampleService, "privateMethod").thenReturn("");
        exampleService.strangeMethod();
    }

As a result of the test I'm getting UnsupportedOperationException. This means, that privateMethod() is called.