Execute a command with Restsharp

Why does the following not work?

dynamic commandMessage = new ExpandoObject();
            commandMessage.command = "work Test 5d";

    RestRequest request = new RestRequest("rest/issue/XXX/execute", Method.POST)
{
      RequestFormat = DataFormat.Json,
    };
    request.AddBody(commandMessage);
    IRestResponse issueResponse = _client.Execute(request);

The Status is OK, but it does Trigger any update on the Server...

Thanks,
Stephan