TCL error message with X-Forwarded-For iRule

Just upgraded from 10.2.0 to 11.4.1. I am using the exact syntax in deb's post from long ago, and I'm receiving a very similar tcl error. Anyone have any ideas?

original post

when HTTP_REQUEST {
  if {[HTTP::header exists X-Forwarded-For]}{
    HTTP::header replace X-Forwarded-For "[HTTP::header X-Forwarded-For], [IP::client_addr]"
  } else {  
    HTTP::header insert X-Forwarded-For [IP::client_addr]
  }
}

tcl error in my logs:

Apr 17 14:59:01 bigip2 err tmm[14652]: 01220001:3: TCL error: /Production/rule_remove_and_insert_XFF_header  - Operation not supported (line 1)     invoked from within "HTTP::header insert X-Forwarded-For [IP::client_addr]"

Thanks in advance for any help anyone can provide. I'm not finding anything else related.