iRule for cookie tracking

I wrote a simple iRule to read the cookie contents in Persistence Cookie Insert mode. Some times, however, the result of the log is an empty string in the HTTP_RESPONSE
Is this normal?

when HTTP_REQUEST { log local0. "ClntIP: [IP::client_addr]:[TCP::client_port] Client values [HTTP::cookie value BIGipServerpool-sPs]" }
when HTTP_RESPONSE { log local0. "SrvIP: [IP::server_addr] Server values [HTTP::cookie value BIGipServerpool-sPs]]" }