ITfoxtec IDX13102: Exception thrown while reading 'AuthnContext' for Saml2SecurityToken

Question

I'm getting the following exception while trying to read the SAML assertion using ITfoxtec Identity SAML2 MVC Core package v4.10.7 in an ASP.NET core 3.1 application:

IDX13102: Exception thrown while reading 'AuthnContext' for Saml2SecurityToken. IDX10000: The parameter 'value' cannot be a 'null' or an empty object. (Parameter 'value')

The error occurs while trying to read the SAML response (3rd line below) in ACS:

var binding = new Saml2PostBinding();
var saml2AuthnResponse = new Saml2AuthnResponse(config);
binding.ReadSamlResponse(Request.ToGenericHttpRequest(), saml2AuthnResponse);

I tried using the SAML Tracer for Chrome and the SAML response appears to be coming OK there even as I saw the 500 Internal Server error.

I'm not sure what value is null in AuthContext. Here's my AuthContext from the SAML tracer:

<saml2:AuthnStatement
    xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion"
                              AuthnInstant="2024-01-05T20:20:08.069Z"
                              SessionIndex="REDACTED"
                              >
    <saml2:AuthnContext>
        <saml2:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport</saml2:AuthnContextClassRef>
    </saml2:AuthnContext>
</saml2:AuthnStatement>

I tried playing around with the Saml2 config, but can't make it work. Currently, my Saml2 config looks like this:

"Saml2": {
    "IdPMetadata": "https://dish.okta.com/app/REDACTED/sso/saml/metadata",
    "Issuer": "http://www.okta.com/REDACTED",
    "SignatureAlgorithm": "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256",
    "CertificateValidationMode": "ChainTrust",
    "RevocationMode": "NoCheck",    
    "SigningCertificateFile": "okta.cer"
  }

I'm stuck. Please help!