arial isn't allowed?

Hello W3.org,

when I try to validate my CSS-file, the validator says " font-family : arial, sans-serif;" isn't good CSS.
"It contains too many (unknown) values".
But when I delete 'arial, ' it is OK.
So the 'unknown value' is arial, but this is a standard font...

Greetings,
René van den Berg from the Netherlands