iRules randomly generating error

This is on an older 9.4.3 that we will be replacing in the next month or so but I have an iRule that throws errors randomly. I can't figure out why. We see allot of traffic on these vips and it will throw this error perhaps 5% of the time.

Nov 29 05:19:40 tmm tmm[1676]: 01220001:3: TCL error: X-Forwarded-For - Operation not supported (line 1) invoked from within "HTTP::header insert X-

This is my iRule;

when HTTP_REQUEST {

Any input would be appreciated is this is enough to spam our logs and perhaps causing problems that we have not discovered yet.

Thanks

Pat