IRULE ERROR invoked from within "HTTP::collect $req_length"

Hi guys,

In LTM Log's i see a error witch a irule.

Do you know what it can be?

The function of the irule what is consumed http and send to another server.

Irule code (Line error)

if { [HTTP::header Content-Length] > 0 } then {
    set req_length [HTTP::header "Content-Length"]
    HTTP::collect $req_length
} else {
    set req_length 4000000
}

BIG IP ERROR IN LOG Mar 31 08:52:08 balancexxx err tmm[14222]: 01220001:3: TCL error: /Common/irule_siem_TIVIT - Operation not supported (line 14) invoked from within "HTTP::collect $req_length"

Thank you,