Using user-extension.js in Selenium RC

I'm having a hard time using extensions.js in Selenium RC.

My Java code,

import com.thoughtworks.selenium.*;
import junit.framework.*;
import java.lang.String;

public class javaErrorCheck extends SeleneseTestCase
{
    Boolean Result=false;
    String S=null;
    String Host="localhost";
    String Timeout="30000";

    public void setUp() throws Exception
    {
        setUp("http://"+Host+"/", "*iexplore");
    }

    public static Test suite()
    {
        return new TestSuite(javaErrorCheck.class);
    }
    public static void main(String args[])
    {
        junit.textui.TestRunner.run(suite());
    }

    public void test_Login() throws Exception
    {
        selenium.setTimeout(Timeout);
        selenium.open("http://"+Host+"/LoginPage.do");
        selenium.windowFocus();
        selenium.windowMaximize();
        selenium.windowFocus();
        selenium.getEval("doDisplayAlert('testing','test');");
        selenium.type("userName", "admin");
        selenium.type("password", "admin");
        Result=selenium.isElementPresent("//input[@type='image']");
        selenium.click("//input[@type='image']");
        selenium.waitForPageToLoad(Timeout);
    }

}

      

        
        
        
      

    

my user-extensions.js

Selenium.prototype.doDisplayAlert = function(value, varName)
{
    alert(value);

}; 

      

        
        
        
      

    

When I run this with the getEval () method this error was thrown,

1) test_Login(javaErrorCheck)*com.thoughtworks.selenium.SeleniumException:
ERROR: Threw an exception: Object expected*
    at
com.thoughtworks.selenium.HttpCommandProcessor.throwAssertionFailureExceptionOrError(HttpCommandProcessor.java:97)
    at
com.thoughtworks.selenium.HttpCommandProcessor.doCommand(HttpCommandProcessor.java:91)
    at
com.thoughtworks.selenium.HttpCommandProcessor.getString(HttpCommandProcessor.java:262)
    at
com.thoughtworks.selenium.DefaultSelenium.getEval(DefaultSelenium.java:443)
    at javaErrorCheck.test_Login(javaErrorCheck.java:34)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at
com.thoughtworks.selenium.SeleneseTestCase.runBare(SeleneseTestCase.java:212)
    at javaErrorCheck.main(javaErrorCheck.java:23) 

      

        
        
        
      

    

If I use runScript (), the expected object error was thrown in the browser as a JS error. In both cases, a warning is displayed in the RC window. I think selenium was unable to gain control after executing the command.

Any ideas?