[jQuery] BlockUI: What's wrong with this code?

[jQuery] BlockUI: What's wrong with this code?

Can anyone tell me what's wrong with the following code:
            $(function(){
                $(".thumb").bind("mouseover", function(){
                    $(this).blockUI({
                        message: $(this).attr("alt")
                    })
                }).bind("mouseout",function(){
                        $(this).unblockUI();
                });
            });
the "thumb" class is applied to a bunch of images on my page. when I mouseover the image I want a layer put up over just that image along with a message (the alternate text). The error I get is: $(this).blockUI() is not a function. Likewise I get the same on the mouseout only with the appropriate function name listed in the error.
Can anyone tell me what I'm doing wrong?
Thanks!
Chris<br clear="all">
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