[akkartik]

It's worked for me for years: http://i.imgur.com/bUB3u.png

[Hurdy]

This works because this filter will match every mail, not just the mails that have the spam label. In the end it does what you wanted but the error is still right.

[tghw]

Odd. I've never gotten it to work. I tried again today just to see if they may have changed that, but still gives me the error.

[akkartik]

I think I remember seeing the message actually. You just blow past it and setup the filter, and it'll work, I think.