Make ASP.NET WCF convert dictionary to JSON, omitting "Key" & "Value" tags

Here's my dilemma. I'm using a RESTful ASP.NET service, trying to get a function to return a JSON string in this format:

{"Test1Key":"Test1Value","Test2Key":"Test2Value","Test3Key":"Test3Value"}

But I'm getting it in this format instead:

[{"Key":"Test1Key","Value":"Test1Value"},
{"Key":"Test2Key","Value":"Test2Value"},
{"Key":"Test3Key","Value":"Test3Value"}]

My method looks like this:

[OperationContract]
[WebInvoke(Method = "POST", BodyStyle = WebMessageBodyStyle.WrappedRequest, RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json)]
public Dictionary<string, string> Test(String Token)
{
    if (!IsAuthorized(Token))
        return null;

    if (!IsSecure(HttpContext.Current))
        return null;

    Dictionary<string, string> testresults = new Dictionary<string, string>();
    testresults.Add("Test1Key", "Test1Value");
    testresults.Add("Test2Key", "Test2Value");
    testresults.Add("Test3Key", "Test3Value");
    return testresults;
}

Is there any way I can get rid of those "Key" and "Value" tags using only built in ASP.NET tools? (i.e., I'd rather not use JSON.NET, if it's avoidable)

Thanks very much! :)