Null Pointer Exception while mocking a method stub

I have a unit test method (testMethodToTest) which internally calls helperMethod .So I try to mock using Mockito.when

I have tried anyList() as well, but still same issue.

@Test
public void testMethodToTest() throws Exception {
        List<Long> workqueues = new ArrayList<>();
        workqueues.add(10000L);

        List<String> tasks=new ArrayList<>();
        tasks.add("Task_1v9et4t");
        Mockito.when(service.helperMethod(workqueues)).thenReturn(tasks);
Map<String, Object> result = service.methodToTest(workqueues) );
    }

I keep getting Null Pointer Exception in Mockito.when() line.