MRUnit reducer test: Mismatch in value class

My reducer unit test throws a "Mismatch in value class" exception after the migration to MapReduce 2:

Mismatch in value class: expected: class org.apache.hadoop.io.IntWritable actual: class com.company.MyWritable

The error message itself is clear to me, but I don't understand why MRUnit gets the temporary writable class instead of IntWritable.

The reducer implementation:

public static class TestCountReduce extends
        Reducer<Text, MyWritable, Text, IntWritable> {

    public void reduce(Text key, Iterator<MyWritable> values,
            Context context) throws IOException, InterruptedException {

        ...
        context.write(key, new IntWritable(s.size()));
    }
}

The test setup:

public void setUp() throws IOException {
    Mapper<Object, Text, Text, MyWritable> mapper = new MyMapper();
    Reducer<Text, MyWritable, Text, IntWritable> reducer = new MyReducer();

    mapDriver = new MapDriver<Object, Text, Text, MyWritable>();
    mapDriver.setMapper(mapper);

    reduceDriver = new ReduceDriver<Text, MyWritable, Text, IntWritable>();
    reduceDriver.setReducer(reducer);
}

And finally the test case:

@Test
public void testReducer() throws IOException {
    List<MyWritable> values = new ArrayList<MyWritable>();
    values.add(new MyWritable("1"));
    values.add(new MyWritable("1"));
    reduceDriver.withInput(new Text("testkey"), values);
    reduceDriver.withOutput(new Text("testkey"), new IntWritable(1));
    reduceDriver.runTest();
}

Please check the reduce method signature in your reducer implementation

It should be

public void reduce(Text key, Iterable<MyWritable> values, Context context) throws IOException, InterruptedException {

instead of

public void reduce(Text key, Iterator<MyWritable> values, Context context) throws IOException, InterruptedException {