NGeo.GeoNames.GeoNamesContainer.LookupPostalCode C# (CSharp) Method

        public ReadOnlyCollection<PostalCode> LookupPostalCode(PostalCodeLookup lookup)
        {
            if (lookup != null) lookup.UserName = _userName;
            return _client.LookupPostalCode(lookup);
        }

 public void GeoNames_LookupPostalCode_ShouldReturnNull_WithoutUserName()
 {
     using (var geoNames = new GeoNamesContainer(null))
     {
         var finder = new PostalCodeLookup();
         var results = geoNames.LookupPostalCode(finder);
         results.ShouldBeNull();
     }
 }