Stupid IE

For some reason the top borders of the tabs won’t show up in Internet Explorer. It does work in Mozilla and Opera, but not in IE. And I don’t understand why, what’s wrong with this?

.activetab {
padding-left: 9px;
padding-right: 9px;
padding-top: 3px;
padding-bottom: 30px;
border-style: solid;
border-top-width: 1px;
border-left-width: 1px;
border-right-width: 1px;
border-bottom-width: 0px;
border-color: #000000;
background-color: #ffffff;
}