Pengecualian Jackson yang Tidak Tertangani?

Saya baru mengenal pemrograman Android, dan saya mengikuti tutorial ini untuk membuat Program server GCM. Namun, saya menemukan bug yang membuat frustrasi dan sangat menghargai bantuan apa pun.

Ini adalah kelas POST2GCM saya:

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import com.fasterxml.jackson.databind.ObjectMapper;


public class POST2GCM extends Content {

private static final long serialVersionUID = 1L;

public static void post(String apiKey, Content content){

        try{

        // 1. URL
        URL url = new URL("https://android.googleapis.com/gcm/send");

        // 2. Open connection
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();

        // 3. Specify POST method
        conn.setRequestMethod("POST");

        // 4. Set the headers
        conn.setRequestProperty("Content-Type", "application/json");
        conn.setRequestProperty("Authorization", "key="+apiKey);

        conn.setDoOutput(true);

            // 5. Add JSON data into POST request body

            //`5.1 Use Jackson object mapper to convert Contnet object into JSON
            ObjectMapper mapper = new ObjectMapper();

            // 5.2 Get connection output stream
            DataOutputStream wr = new DataOutputStream(conn.getOutputStream());

            // 5.3 Copy Content "JSON" into
            mapper.writeValue(wr, content);

            // 5.4 Send the request
            wr.flush();

            // 5.5 close
            wr.close();

            // 6. Get the response
            int responseCode = conn.getResponseCode();
            System.out.println("\nSending 'POST' request to URL : " + url);
            System.out.println("Response Code : " + responseCode);

            BufferedReader in = new BufferedReader(
                    new InputStreamReader(conn.getInputStream()));
            String inputLine;
            StringBuffer response = new StringBuffer();

            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();

            // 7. Print result
            System.out.println(response.toString());

            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
}

Saya telah menyertakan file "jackson-databind-2.5.1.jar" tetapi saya mendapatkan kesalahan:

Pengecualian yang Tidak Tertangani: com.fasterxml.jackson.databind.JsonMappingException

di telepon mapper.writeValue(wr, content);

Apa yang menyebabkan pengecualian ini, dan bagaimana cara memperbaikinya?