request.LogonUserIdentity.AuthenticationType value is empty

I use a control statement for the authentication
if(request.LogonUserIdentity.AuthenticationType == "NTLM" || request.LogonUserIdentity.AuthenticationType == "Negotiate"){ ... }
where the value of request.LogonUserIdentity.AuthenticationType during debugging is always an empty string.
I used the following command " cscript adsutil.vbs get w3svc/NTAuthenticationProviders " to verify that IIS is configured correctly, indeed the output was:
Microsoft (R) Windows Script Host Versione 5.6
Copyright (C) Microsoft Corporation 1996-2001. Tutti i diritti riservati.

NTAuthenticationProviders       : (STRING) "Negotiate,NTLM"
Anyone has some insight about the reason why I get an empty value for request.LogonUserIdentity.AuthenticationType?
Thanks in advance





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