Error using :after

Suddenly the W3C CSS validator is complaining about a statement that I 
do not believe it has complained about before.

Statement:
    .pageno:after { color: silver; content: attr(title); }

Error message:
18  .pageno:after  Value Error : content 
<http://www.w3.org/TR/2011/REC-CSS2-20110607/generate.html#propdef-content> 
Parse Error attr(title)


I'm validating as CSS 2.1. Is that statement really invalid for some 
reason I don't understand, or has a recent change to the validator 
introduced this behavior in error?

Thanks,
Walt