JQuery :: Ajax Invalid Json

client code:

<script src="../Scripts/jquery-1.4.1.min.js" type="text/javascript"></script>
<script type="text/javascript" language="javascript">
$("#send").click(function () {
$.ajax({
url: "../JqueryAjax/JqueryAjaxServer.aspx",
type: 'get',
contentType: "application/json; charset=utf-8",
dataType: "json",
timeout: 30000,
data: "{}",
success: function (data, textStatus) {
alert("success");
},
error: function (XMLHttpRequest, textStatus, errorThrown) {
alert("error");
alert(textStatus);
alert(errorThrown.toString());
}
});
});
</script>

server code:

Response.ContentType ="application/json";
string json = "{"username":"test"}";// ,"content":"test"
Response.Write(json);

when i click the post request button the result is shown like this Invalid Json,the browser alert this.