Unsuccessful matching against "address" data group

I'm trying to block IPs that show up in the X-Forwarded-For header, and I'm quickly finding out that the "Address" data group is very picky as to what you try to compare against it.

The rule I tried:

when HTTP_REQUEST {
  if { not ([HTTP::header values "X-Forwarded-For"] equals "") } {
    if { [matchclass [HTTP::header values "X-Forwarded-For"] equals $::block_ip] } {
      log local0.info "Blocking: [IP::remote_addr]"
      HTTP::respond 410
    }
  }
}

My first problem was that "X-Forwarded-For" wasn't always set. When it wasn't, "[matchclass [HTTP::header values "X-Forwarded-For"] equals $::block_ip]" would throw errors. So that's when I put in the equals "" part. Now, that works fine until the X-Forwarded-For contains anything other than a single IP.

The error I get is:

TCL error: iRule_BlockIP - bad IP address formatInvalid class element for class block_ip invoked from within "matchclass [HTTP::header values "X-Forwarded-For"] equals $::block_ip"

So that's where I'm at. Should I give up on using the "Address" data group? Can anyone tell me of a better way to do this?

Many thanks