conditional redirect (404)

Hi people,

I´m trying to wrote a i-rule to redirect a 404 error based on the url. The i-rule I made is this one:

when HTTP_REQUEST { 
 set host [HTTP::host] 
 set url  [HTTP::uri] 
 } 
 when HTTP_RESPONSE { 
 set status_code [HTTP::status] 
 if { $status_code equals 404 } { 
 if { $host contains "abcd" } { 
 HTTP::redirect "http://abcd.otherdomain.com/$url" 
 } 
 elseif { $host contains "efg" } { 
 HTTP::redirect "http://efg.otherdomain.com/$url" 
 } 
 else {  
 HTTP::redirect "http://anotherone.com/xyz.html" 
 } 
   }   
 }

it seens ok to me and it works almost every time, but sometimes I received the following error:

Jun  9 19:25:07 tmm tmm[1133]: 01220001:3: TCL error: Rule irule_redirect_404  - can't read "host": no such variable     while executing "if { $host contains ..."

any ideas why the host variable doesn´t work sometimes?