Error with jsonpCallback. Internal Server Errror
===

First of all I get in position. I'm trying to access ajax (jquery-mobile and PhoneGap) to a web service (asmx). 
This is the js code:

    $.ajax({
    	type		: "POST",
        url	        : webMethod,
        data		: param,
    	dataType	: 'jsonp',
        jsonp		: 'callback',
        jsonpCallback   : 'jsonpCallback',
    	success		: function(data) {  
    	  		       alert(data.nombre);  
    	        	  },
        error		: function(x,y,z) {
        		       alert("-- " + x+'\n'+y+'\n'+z + " --");
    			  }
        });
    	    
        function jsonpCallback(){
        	alert("OK");
        }
    

and the web service (asmx)

    [WebMethod]
    //public string ObtenerOfertas()
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public void ObtenerOfertas()
    {
      HttpContext.Current.Response.ContentType = "application/json";
      string qs = HttpContext.Current.Request.QueryString["jsonpCallback"];
      HttpContext.Current.Response.Write(qs + "( [{ \"name\": \"John\", \"message\": \"Ok\"}] )");
    }

When I run the debug gives me an error --> 

callback=jsonpCallback&_=1343139529496 500 (Internal Server Error) 

I have already tried a thousand things (or at least that's the feeling I have ...)

I would greatly appreciate your suggestions.

P. S. Forgive my poor English