irule cookie insert

I had modify the iRules and the persistence works for a single user. However, when I do a load test with 10 users, I am hit by tcl error on iRules stating the following:

- can't read "cookie_value": no such variable while executing " HTTP::cookie insert name webserver value $cookie_value"

Modify irules as the following:

when HTTP_REQUEST {
pool  web_pool
  if { [HTTP::cookie exists webserver] } {
    persist uie [HTTP::cookie value webserver]
  }
}
when HTTP_RESPONSE {
  switch [IP::server_addr] {
    172.10.20.1 { set cookie_value "webA" }
    172.10.20.2 { set cookie_value "webB" }
  }
  HTTP::cookie insert name webserver value $cookie_value
  persist add uie $cookie_value
}

The load test result was terrible with 50% errors flag out.