ajax call returns null in my facebook iframe

I am using jquery to send an ajax request from my facebook app which is in iframe to my server. The ajax request works fine when the web app is running stand alone and out of facebook platform, but within facebook, the result that I get from my ajax re开发者_运维问答quest is blank !!! Here is the code I use:

$.ajax({
    url: 'http://mydomain.com/search',
    data: params,
    cache: false,
    dataType: 'json',
    success: function(posts) {
                // post is null !!!!!
             }
    error: function(json) {
                alert('error');
            }            
    });

I appreciate if anybody can help me with it.

Thanks.

uhsp

Ok I solved it myself. It was a cross domain issue.