Nexmo C# Lib giving 'Missing username' error

Question

I am using nexmo c# lib and using below code

var results = SMS.Send(new SMS.SMSRequest
                               {
                                   from = smsOptions.From,
                                   to = smsOptions.To,
                                   text = smsOptions.Text
                               });

added below keys in web.config:

<add key="Nexmo.UserAgent" value="myApp/1.0" />
<add key="Nexmo.Url.Rest" value="https://rest.nexmo.com" />
<add key="Nexmo.Url.Api" value="https://api.nexmo.com" />
<add key="Nexmo.api_key" value="..." />
<add key="Nexmo.api_secret" value="..." />

Any help would be highly appreciated.