Javascript - How To Use MS Ajax PageMethods To Call A Function On The Server

I have a project that uses PageMethods to call functions on the server. The server functions (written in C#) return the values as array of strings, without doing any kind of serialization and in the client side (from Js) the accessing of the return values is by using static variable called arguments. I found that sometimes for some users (cases are not repro) sometimes an exception occured

"WebServiceFailedException the server method 'Foo' returned invalid data. the 'd' property is missing from JSON."

Some searching on google I found that people are serializing the return values using DataContractJsonSerializer class and in js accessing the return value using one of the callback function Example:

function OnRequestComplete(result,
userContext, methodName) {
var Person = eval('(' + result + ')');
alert(Person.Forename);
alert(Person.Surname); }

So is the first technique is correct? or what?

P.S:

the function on the server is defined on the default.aspx.cs file as follows:

[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
public static string[] Foo(string s);

from the client side the calling is as follows

PageMethods.Foo("value",OnSuccess);

Also all the users have the same browser version (IE8)