Using the DynamicDictionary - adamralph/scriptcs-nancy GitHub Wiki

At the time of writing, due to restrictions with dynamic types in Roslyn, Nancy's DynamicDictionary cannot be used as advertised.

Instead, use this workaround:

Get["/goodbye/{name}"] = parameters => {
    return "Goodbye " + ((IDictionary<string, object>)parameters)["name"];
};