pam_radius requires setting Auth-Type ?

[NdK]

Hello all.

We just added pam_radius to our vpn host, to authenticate vpn users
throught our (working) RADIUS server.

IIUC pam_radius is sending a PAP message:
Access-Request packet from host 192.168.130.61 port 9327, id=233,
length=99
	User-Name = "STUDENTI\\studente.fittizio"
	User-Password = "my-cleartext-password"
	NAS-IP-Address = 130.136.152.6
	NAS-Identifier = "openvpn"
	NAS-Port = 8302
	NAS-Port-Type = Virtual
	Service-Type = Authenticate-Only

But if I don't add (in users file) a line like:
DEFAULT NAS-Identifier == "openvpn", Auth-Type := PAP
FR complains:
ERROR: No authenticate method (Auth-Type) found for the request:
Rejecting the user

IIUC, Auth-Type should never be set manually, so I'm quite sure I'm
missing something...
Could you please point me in the right direction?

Tks.

BYtE,
 Diego.