Button Click to Drop Menu Behind Fixed Header

Trying to create a menu that drops behind a fixed header. Right now, it drops under the button, but above the header, despite z-indexing.

HTML

<header>
    <div class='menu-wrap'>
    <div id="menu-button" class="up">menu</div>
    </div>
    <nav role="primary" class="hide">
        <ul>
            <li><a href="#">foobar</a></li>
            <li><a href="#">foobar</a></li>
            <li><a href="#">foobar</a></li>
            <li><a href="#">foobar</a></li>
            <li><a href="#">foobar</a></li>
            <li><a href="#">foobar</a></li>
        </ul>
   ...

CSS

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
}
header {
    display: block;
    float: left;
    width: 100%;
    height: 50px;
    background-color: hsla(0, 0%, 20%, 1);
    box-shadow: 0px 2px 8px #222;
    position: fixed;
    top: 0;
     xposition:relative;
}

.menu-wrap {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: blue;
}
#menu-button {
    display: block;
    float: left;
    background-color: hsla(0, 0%, 50%, 1);
    color: #fff;
    border: 2px solid #222;
    border-radius: 6px;
    padding: .25em .5em;
    margin-left: 10px;
    margin-top: 10px;
    cursor: pointer;
}
.up {
    background-image: linear-gradient(hsla(0, 0%, 100%, .2), hsla(0, 0%, 0%, .2));
}
.down {
    background-image: linear-gradient(hsla(0, 0%, 0%, .2), hsla(0, 0%, 100%, .2));
}
nav {
    position: relative;
    z-index: -4;
}
nav ul {
    list-style:none;
    width: 100%;
    position: absolute;
    top: 50px;
    xz-index: -4;
    transition: all .6s ease;
    -webkit-transition: all .6s ease;
    -moz-transition: all .6s ease;
    -o-transition: all .6s ease;
}
.hide ul {
    top: -500px;
}
.reveal ul {
    top: 50px;
}
nav ul li a, nav ul li a:visited {
    display: block;
    float: left;
    width: 50%;
    background-color: hsla(0, 0%, 35%, 1);
    text-decoration: none;
    text-align: center;
    color: #fff;
    padding: 1em;
    border-right: 1px solid #fff;
    border-bottom: 1px solid #fff;
    transition: .4s all;
    -webkit-transition: .4s all;
    -moz-transition: .4s all;
    -o-transition: .4s all;
}
nav ul li:nth-child(even) a {
    border-right: none;
}
nav ul li:hover a {
    background-color: hsla(0, 0%, 50%, 1);
}

JavaScript

$('#menu-button').click(function () {
    $(this).toggleClass('down');
    $('nav').toggleClass('reveal');
});