Prerequisite operation not in progress

Can anyone suggest what this error message is about. It has been appearing intermittently since I added the rule below. It seems to be upset with the "set uri [string tolower [HTTP::uri]]" line.

They're running v9.1.1

Sep 28 14:13:14 tmm tmm[728]: 01220001:3: TCL error: Rule dhs-common-http-rule - Prerequisite operation not in progress (line 1) invoked from within "HTTP::uri"

virtual dhs-common-http-vs {
   destination 111.111.111.111:http
   ip protocol tcp
   profile http tcp
   pool dhs-common-http-pool
   rule dhs-default-rule dhs-common-http-rule
}
rule dhs-common-http-rule {
   timing on
  when HTTP_REQUEST {
        set uri [string tolower [HTTP::uri]]
        if { [matchclass $uri equals $::dhs_common_equals_class]
             or [matchclass $uri starts_with $::dhs_common_prefix_class]
             or [matchclass $uri ends_with $::dhs_common_suffix_class]
             or [matchclass $uri contains $::dhs_common_contains_class] } {
           return
        }
       log "DHS Reject: [HTTP::host][HTTP::uri]"
  }
}