JSONP request in chrome extension, callback function doesn't exist?

I am making a JSONP request in a chrome extension (content script) . Everything works very well when I am running as a webpage -loading the HTML file in my browser-, but when I load it as a chrome extension, the jsonp callback function created by jquery doesn't seem to exist when the server gives its response.

My console says:

Uncaught ReferenceError: jQuery17105683612572029233_1323808231542 is not defined

Here is my ajax request:

$.ajax({
    url: 'http://example.com',
    data: 
    {
        imgUrl: this.href,
        returnString:true
    },
    dataType: "jsonp",
    success: function(msg){
        newNode.src = msg.data;
    },
    error: function(msg){
        console.log(msg.data);
    }
})