Can I Set the HTTP Response Code & Throw an Exception on an ASMX JSON Service?

In an ASP.NET ASMX WebMethod that responds JSON, can i both throw an exception & set the HTTP response code? I thought if i threw an HttpException, the status code would be set appropriately, but it cannot get the service to respond with anything but a 500 error.

I have tried the following:

[WebMethod]
[ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
public void TestWebMethod() {
    throw new HttpException((int)HttpStatusCode.BadRequest, "Error Message");
}

Also:

[WebMethod]
[ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
public void TestWebMethod() {
    try {
        throw new HttpException((int)HttpStatusCode.BadRequest, "Error Message");
    }
    catch ( HttpException ex ) {
        Context.Response.StatusCode = ex.GetHttpCode();
        throw ex;
    }
}

These both respond with 500.

Many thanks.