CSS dropdown closes too early - html

Question

My dropdown works fine, except it closes before i can move mouse top of last link in it.
CSS
nav {
width: 100%;
display: inline-block;
margin-bottom: 1%;
text-align: center;
background-color: #F0F0F0;
}
nav .links {
width: 100%;
line-height: 1.2;
font-size: 100%;
text-decoration: underline;
text-align: center;
}
nav .links a {
color: #666666;
text-decoration: none;
word-spacing: normal;
}
nav .links a:visited {
color: #666666;
text-decoration: none;
}
nav .links a:hover {
color: #383838;
text-decoration: none;
}
nav .links a:active {
color: #666666;
text-decoration: none;
}
nav ul {
position:relative;
list-style:none;
color: #666666;
white-space: nowrap;
}
nav li{
position:relative;
float: left;
margin-left: 5%;
}
nav ul li ul {
display: none;
position: absolute;
background-color: #F0F0F0;
border: 2px solid;
border-radius: 5px;
padding: 0.5em 1em 0.5em 0.5em;
line-height: 1.2;
}
ul li:hover ul {
display: block;
}
HTML
<nav>
<div class="links">
<ul>
<li>ETUSIVU</li>
<li>HENKILÖKUVA JA HISTORIA</li>
<li>KORISTEKERAMIIKKA</li>
<li>GALLERIA
<ul>
<li>Keramiikkaveistokset</li>
<li>Keramiikka - kuparityöt</li>
<!--Next link is the one where dropdown closes before mouse reaches it-->
<li>Krisu testi</li>
</ul>
</li>
</ul>
</div>
I tested it with Chrome and FF. CSS is not top skill for me and i think this should work but obviously im wrong :)
I would appreciate help with this greatly, thanks.
EDIT
I changed nav ul li ul as...
nav ul li ul {
z-index: 1;
display: none;
position: absolute;
background-color: #F0F0F0;
border: 2px solid;
border-radius: 5px;
padding: 0.5em 1em 0.5em 0.5em;
}
and now it works just fine. So basically i just added z-index there.
There is an image right below dropdows, not sure is it possible that it messes this one? Atleast z-index did help...