Exception when calling Okta authentication

I'm trying to use Okta to authenticate to my application that doesn't need SSO, just to authenticate user data to Okta, then go ahead and check other systems if that fails. APIs are a bit sparse, but I try to do the following

var oktaClient = new OktaClient("my-api-token", new Uri("https://mysubdomain.okta.com"));
var authClient = oktaClient.GetAuthClient();
var oktaResponse = authClient.Authenticate(username, password);

      

        
        
        
      

    

This last line is throwing Okta.Core.OktaException. Details:

Error making an HTTP request: One or more errors occurred.
Additional information: Error making an HTTP request: One or more errors occurred.