 <p>It seems your mapper outputs a key-value pair of type <code>ImmutableBytesWritable -&gt; Result</code> but your Reducer reads in a key-value pair of type <code>ImmutableBytesWritable -&gt; Put</code>. This can't work as the output of the mapper must be of the same type the input of the reducer is. You should either</p> <ul> <li>change your mapper to output a <code>Put</code> object and adjust your method and class signatures</li> <li>or change the reducer signatures to allow for a Result object as in-value.</li> </ul>