Help: Radius Accounting Request and Message Authenticator

[Alessio Grasso]

Hi all, 
I'm a ICLoS's software engineering and I'm developing a WiMax ASN. 
I had already develop the client authentication and authorization modules for freeradius 2.1.8, successfully (EAP-TTLS). Now I'm building the Radius Accounting module but I have a problem with the Request-Authenticator find in the message of Accounting-Request. When I send a Radius Accounting-Request, the server's log shows:

Received packet from 192.168.2.130 with invalid Message-Authenticator! (Shared secret is incorrect.) Dropping packet without response.

I think it's wrong to calculate HMAC-MD5 (is it exactly the same to the case of Authentication Request?). I'm studing on the RFC 2866 but I found that there are only 16 octets to zero in the Authenticator field.
Where can I found further informations?

Thanks in advance,

Alessio Grasso