Problems with header rule

[Matthias Keller]

Hi

I want to take an upstream spamfilter into account.
If it thinks it's spam it adds the following header to the mail:
X-Spam-Classification: spam

But my rule
header  MKE_UP_SPAM         X-Spam-Classification =~ /spam/i
describe MKE_UP_SPAM        upstream spamfilter thinks it's spam
score   MKE_UP_SPAM         1.0

never matches... ? Is there a problem with the dashes in the header name 
or ... ??

Thanks

Matt