Anyone knows why I keep getting following error message?

Aug  2 13:10:26 localhost openser[30695]: ERROR: extract_body: message body has length zero

Aug  2 13:10:26 localhost openser[30695]: ERROR: force_rtp_proxy2: can't extract body from the message

I turn on nathelper,  and saw from the following source code:

int extract_body(struct sip_msg *msg, str *body )

{

        body->s = get_body(msg);

        if (body->s==0) {

                LOG(L_ERR, "ERROR: extract_body: failed to get the message body\n");

                goto error;

        }

        body->len = msg->len -(int)(body->s-msg->buf);

        if (body->len==0) {

                LOG(L_ERR, "ERROR: extract_body: message body has length zero\n");

                goto error;

        }

Why do we care about the content-length, which normally is 0 for most of the SIP UAs?   Following is an example SIP UA Registration and you can see “Content-Length: 0” in the bottom:

Session Initiation Protocol

    Request-Line: REGISTER sip:sip.mycompany.com SIP/2.0

        Method: REGISTER

        Resent Packet: True

        Suspected resend of frame: 62339

    Message Header

        Via: SIP/2.0/UDP 192.168.254.6:5062;branch=z9hG4bK0997e90280d2f8e5

        From: "Bill Office Phone" <sip:8001@sip.mycompany.com>;tag=ec200436c1169e42

            SIP Display info: "Bill Office Phone"

            SIP from address: sip:8001@sip.mycompany.com

            SIP tag: ec200436c1169e42

        To: <sip:8001@sip.mycompany.com>

            SIP to address: sip:8001@sip.mycompany.com

        Contact: <sip:8001@192.168.254.6:5062>

        Call-ID: 5a2ae3a1b883568f@192.168.254.6

        CSeq: 20113 REGISTER

        Expires: 120

        User-Agent: ABC 1.0.0.0

        Max-Forwards: 70

        Allow: INVITE,ACK,CANCEL,BYE,NOTIFY,REFER,OPTIONS,INFO,SUBSCRIBE,UPDATE,PRACK,MESSAGE

        Content-Length: 0

Thanks in advance.

Best Regards,
Bill Zhang