nested elseif unexpected behaviour

I guess I'm trying to make too many changes at once so I decided to narrow my focus. First I need to get the rule to work with Data Groups. So I used Kevin's rule and checked it against my environment:

    when HTTP_REQUEST {
    if { [class match [string tolower [HTTP::uri]] starts_with QA-HEC-ENT-URI] } {
        pool INF-ENT-HTTP
    } elseif { [class match [string tolower [HTTP::uri]] starts_with QA-HEC-URI] } {
        pool INF-HEC-DNNREDIRECT
    } else { 
        pool INF-HEC-HTTP
    }
}

Now I get a 500 Internal server error. I'm not familiar with HTTPWatch or Fiddler so I'm going to have to introduce myself to them before I can see what's going on under the hood.