inserting client ip address into header when using SNAT

Any info on the error that we are getting? We consistently get the PREPEND_HEADERS error when attempting the TXT file download. We originally thought the extra 2 bytes at the end of the content was causing a problem with BigIP delivering the file, but this following iRule failed to correct the problem.

  
  rule handle_two_trailing_bytes {  
     when HTTP_REQUEST {  
        set two_byte_error 0  
        if { [HTTP::uri] contains "export.fetchFile" } {  
           set two_byte_error 1  
        }  
        HTTP::header insert ORIG_CLIENT_IP [IP::remote_addr]  
     }  
     when HTTP_RESPONSE {  
        if { $two_byte_error } {  
           set content_len [HTTP::header Content-Length]  
           incr content_len 2  
           HTTP::header replace Content-Length $content_len  
        }  
     }  
  } 
 

What exactly does the error mean?