I seem to be having a problem with my Wordpress CSS Menu. I am trying to create a dropdown element in the menu, which is conveniently wrapped in a div automatically called "sub-menu".
When normal, the menu looks like this:

However, when I try to access the drop-down menu under "Photography", this happens:

I have tried everything and am unable to get it to correctly show up under Photography. Any help would be much appreciated:
#header li ul.sub-menu {
display:none;
position: absolute;
top: 10px;
left: 50%;
width: auto;
}
#header li:hover ul.sub-menu {
display:block;
}