visible-xs class is identical to visible-sm class

In the bootstrap.css, the visible-xs class is identical to the visible-sm class. visible-xs is missing the following:

@media (min-width: 400px) and (max-width: 767px) {
.visible-xs {
display: none !important;
}
tr.visible-xs {
display: none !important;
}
th.visible-xs,
td.visible-xs {
display: none !important;
}
}