MailUp.Sdk NullReferenceException

I'm trying mailup for the first time and I just installed the nuget package (1.1.7) from https://www.nuget.org/packages/MailUp.Sdk/

And then to give it a try I've tried the first example in their documentation in here

 public static void GetUserInfo(string username, string password)
    {
        // 1) Setup the MailUp Client:
        MailUpClient client = new MailUpClient();

        // 2) Invoke the web method:
        List<UserInfoDTO> result = null;
        try
        {
            result = client.ListUserInfo(username, password);
        }
        catch (Exception ex)
        {
            Console.WriteLine("Cannot perform the operation due to " + ex.Message);
            return;
        }

        // 4) Print results:
        if (result != null)
            Console.WriteLine("Retrieved users total:" + result.Count);
        else
            Console.WriteLine("An error occurred while retrieving users.");
    }

When I run the code simply it throws null reference exception on the MailUpClient client = new MailUpClient(); line. Any ideas?

update 1

Please find the exception details below:

System.NullReferenceException HResult=0x80004003 Message=Object reference not set to an instance of an object. Source=MailUp.Sdk StackTrace: at MailUp.Sdk.Base.MailUpClient..ctor() at mailupTest.Program.GetUserInfo(String username, String password) in C:\Users...source\repos\mailupTest\mailupTest\Program.cs:line 20 at mailupTest.Program.Main(String[] args) in C:\Users...\source\repos\mailupTest\mailupTest\Program.cs:line 14