Why can't I focus a control that I reveal with slideDown()?

$("document").ready(function(){
    $("#MyButton").click(function(e){
    	e.preventDefault();
    	$(".SlidingDiv").slideDown();
    	$("#MyInput").focus();
    });
});

<input type="button" value="Click Me" id="MyButton" />
<div class="SlidingDiv" style="display:none;"><input type="text" id="MyInput" /></div>

$(function(){
    $("#MyButton").click(function(e){
        e.preventDefault();
        $(".SlidingDiv").slideDown(function(){
            // Callback function - will occur when sliding is complete.
            $("#MyInput").focus();
        });
    });
});