C# - Why Does Silverlight Reference To Wcf Service Blow Up When Add Method To The Wcf Service That Returns Generic List

I have built a WCF Service that is being consumed by a Silverlight app. At first I created one method that was very simple:

public String SfTest()
{
return "SF Test";
}

No poblem. My silverlight app references my service and displays "SF Test" in a textbox. Now I add a method to my wcf service like this:

public List<String> GetTest()
{
List<String> list = new List<string>();
String a = "a";
list.Add(a);
String b = "b";
list.Add(b);
return list;
}

I update the reference to the service in my Silverlight app and the using statement in my xaml cs page throws an error like the service doesn't even exist although it is there. I am assuming the problem has to do with datatypes or serialization or something like that but it is driving me up the wall. Why can't I consume a simple generic list in my Silverlight app through the WCF service.