UnnecessaryStubbingException SSCCE

SillyTest.java

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.runners.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class SillyTest {
    static class ClassUnderTest {
        LinearRamp ramp;

        public ClassUnderTest(LinearRamp aRamp) {
            ramp = aRamp;
        }

        int calc(int x) {
            // Un-commenting this breaks the test even though the behaviour is correct
            // if (x == 0) return 0;
            return ramp.compute(x);
        }
    }

    static class LinearRamp {
        int scale;

        LinearRamp(int aScale) {
            scale = aScale;
        }

        int compute(int x) {
            return scale * x;
        }
    }

    final LinearRamp ramp = mock(LinearRamp.class);
    final ClassUnderTest cut = new ClassUnderTest(ramp);

    @Test
    public void testCalcOfNonZero() {
        when(ramp.compute(3)).thenReturn(6);
        assertEquals(6, cut.calc(3));
    }

    @Test
    public void testCalcOfZeroReturnsZero() {
        when(ramp.compute(0)).thenReturn(0);
        assertEquals(0, cut.calc(0));
    }
}