Strip User-Agent from incoming requests

I'm playing around in a lab, and I need to strip the User-Agent from incoming requests. The iRule I created goes like this:

when HTTP_REQUEST { { HTTP::header remove User-Agent } }

However, when I attempt to create the iRule, I receive this error:

1070151:3: Rule [/ISO2/app_strip_useragent] error: /ISO2/app_strip_useragent:2: error: [undefined procedure: HTTP::header remove User-Agent ][{ HTTP::header remove User-Agent }]

Any ideas? Seems to be a pretty simple task, but not sure why it's erroring out...