Campaigns not showing up with Wordpress Setup

Hi guys,

I am running piwik with wordpress and have friendly urls enabled.

My problem is that piwik does NOT track the "?pk_campaign=tester" parameters.
When using it (p.e. www.mydomain.com/page/?pk_campaign=tester) it shows the entry as a "direct entry".

Do you guys have a tip on how to make this work?

Any ideas?

Kind regards
Marcus