selectivizr not working for nth-child(2) in IE7 and IE8

Question

Looking at the resulting CSS in IE developer tools a simple rule such as

.filter-dates .date-picker:nth-child(2)
{
    margin-left:200px;
}

shows up in IE as

UNKNOWN
   margin-left:200px;

We use the latest JQuery lib and according to the Selectivizr site nth-child is supported.

Anyone know where I'm going wrong?

Thanks, Jacques