nth-child bug in validator

Hi,

I'm pretty sure this is a bug... I'm getting the following error in the
validator (http://jigsaw.w3.org/css-validator/validator):

Unknown pseudo-element or pseudo-class :nth-child

pointing to the following:

..codetable td:nth-child(2){

    width: 50px;

    color: blue;

    }


Which, as far as I know, is correct.

Russ