Trimmer plugin breaks magpie

Maybe I am missing something, but using the Trimmer plugin inside my Magpie block breaks Magpie. My code is:

{exp:trimmer left="3" right="4"}{description}{/exp:trimmer}

I am trying to cut the paragraph tags off the ends of the description, but instead I get “script”, which threw me of for a while, until I realized that “script” is “{description}” with the first 3 and the last 4 characters cut off.

So how do I get EE to process the Magpie tag before the Trimmer tag? Am I doing something really stupid here? Thanks.