[jQuery] s.replace is not a function

I borrowed this function from http://sorgalla.com/projects/jcarousel/examples/dynamic_flickr_feed.html

function mycarousel_decodeEntities(s) {
return s.replace(/&/g, "&")
.replace(/"/g, '"')
.replace(/'/g, "'")
.replace(/&lt;/g, "<")
.replace(/&gt;/g, ">");
};

and I'm trying to use it in my code here:

function mycarousel_itemLoadCallback(carousel, state) {

$.getJSON("http://localhost:59396/xxx/xxxHandler.ashx?
action=cjson",
function(data) {
$.each(data, function(i, item) {
carousel.add(i, mycarousel_decodeEntities(item));
if (i == 3) return false;
});
});
};

for some reason I get an error in firebug saying this:

s.replace is not a function
[Break on this error] return s.replace(/&amp;/g, "&")

I know I've used this very same code before and the only difference
was that I changed some JSON that is returned but it's valid json.