Pop up error while using validation in struts2.0

Hi,
I have wriiten a small application using struts 2.0...and then i have tried to implement validation in it.I have given 2 input fields, and when i click on submit without entereing in one of the fields or both, i get a pop-up stating that [object-error].

Kindly, help me out in this...the code is given below...
Thanks in advance.

Login.jsp:

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252"/>
<title>Login</title>

</head>
<body>

<s:form action="loginAction" validate="true">

Username: <s:textfield name="username"/>
Password: <s

assword name="password"/>

<s:submit value="submit"/>
</s:form>
</body>
</html>

loginAction-Validation.xml:

<validators>

<field name="username">

<field-validator type="required">
<param name="fieldName">username</param>



<message>Login name is required</message>

</field-validator>

</field>

<field name="password">

<field-validator type="requiredstring">



<message>Password is required</message>

</field-validator>

</field>

</validators>

loginAction.Java:

public class loginAction extends ActionSupport{

String username;
String password;
public String execute()throws Exception{

System.out.println(""+username);
System.out.println(""+password);
System.out.println("Validating login");
if(!getUsername().equals("Admin") || !getPassword().equals("Admin")){
addActionError("Invalid user name or password! Please try again!");
return "ERROR";
}else{
return "SUCCESS";
}

}

public void setUsername(String username) {
this.username = username;
}

public String getUsername() {
return username;
}

public void setPassword(String password) {
this.password = password;
}

public String getPassword() {
return password;
}
}

does anyone know....how to solve it....
thanks...
Rajiv