CSS3 - border-radius with / gives error

The CSS3 spec for border radius states the value for border-radius can be:
[ <length> | <percentage> ]{1,4} [ / [ <length> | <percentage> ]{1,4} ]?
http://www.w3.org/TR/css3-background/#the-border-radius

But the validator guves an error:
/ is an incorrect operator : 100px/180px  100px/180px
for the line:
    border-radius:100px / 180px;

-Keilin