Why MessageAuthenticator_Set() has to be called twice when calculate HMAC.MD5 hashes?

[taleintervenor]

I have read the issue 27 and successfully add the Message-Authenticator field to request. But I cannot understand why rfc2869.MessageAuthenticator_Set() has to be called twice.

In my own code:

	rfc2869.MessageAuthenticator_Set(packet, make([]byte, 16))
	...
	rfc2869.MessageAuthenticator_Set(packet, hash.Sum(nil))

It seems the second set action should completely overwrite the previous data.
But when I coment out the first MessageAuthenticator_Set, the request sent to freeradius server was dropped with error log like:

Info: Dropping packet without response because of error: Received packet from xxx with invalid Message-Authenticator! (shared secret isincorrect,)

It turns out that the first MessageAuthenticator_Set() is necessary.
So what is the difference it actually made?

[icb-]

Per RFC2869 §5.14

When the checksum is calculated the signature string should be considered to be sixteen octets of zero.

The first call to rfc2869.MessageAuthenticator_Set inserts the attribute in the packet. The second call updates the attribute value to the hash you calculated.