error in calling custom method in auto activity

Hi,
I have written custom method as below
package com.tax;

import java.io.PrintWriter;

import com.documentum.bpm.IDfWorkitemEx;
import com.documentum.bpm.rtutil.WorkflowMethod;
import com.documentum.dmcl.impl.SetPerformersHandler;
import com.documentum.fc.client.DfUser;
import com.documentum.fc.client.IDfActivity;
import com.documentum.fc.client.IDfProcess;
import com.documentum.fc.client.IDfSession;
import com.documentum.fc.client.IDfUser;
import com.documentum.fc.client.IDfWorkflow;
import com.documentum.fc.client.IDfWorkitem;
import com.documentum.fc.common.DfList;
import com.documentum.fc.common.IDfList;
import com.documentum.fc.common.IDfProperties;

public class AssignPerformers extends WorkflowMethod
{
protected int doTask(IDfWorkitem wi, IDfProperties arg1, PrintWriter arg2)

throws Exception

    {

        System.out.println("Inside the SampleAutoActivityMethod ");

if(wi.getRuntimeState() == 0)

         wi.acquire();
        IDfList forwardActivitiesList = wi.getForwardActivities();
        IDfActivity forwardActivity = (IDfActivity) forwardActivitiesList.get(0);
        IDfList performerList = new DfList();

        performerList.setElementType(IDfList.DF_STRING);

        performerList.append("44444");
     // performerList.append("55555");
        wi.setPerformers(forwardActivity.getObjectName(), performerList);
        wi.complete();



        return 0;

}

But when i run the workflow gettting the below error.
Activity Activity-1's Execution failed : '1314714671'
DfList$TypeMismatchForAddException:: THREAD: pool-1-thread-4; MSG: [DM_DFC_E_TYPE_MISMATCH_ADD]error: "Type mismatch. Expected type "int"; got type "Object"."; ERRORCODE: 221; NEXT: null
Email for Activity Activity-1 with Notification Type TASK_FAILED sent from documentum@tcs.com to dmadmin@tcs.com with the message null
Activity Activity-1's workitem Paused : '1314714671'