求助 ajax兼容性（浏览器）问题

[youlanbeilei]

$(document).ready(function() {
$('#checkuser').click(function() {
if ((document.getElementById("<%=txtusername.ClientID%>").value) == "") {
alert('Please input your username ！');

return false;
}
$.ajax({
type: "Post",
url: "CheckUserName.aspx?username=" + document.getElementById("<%=txtusername.ClientID%>").value,
data: "{}",
datatype: "text",
success: function(text) {
alert(text);
},
error: function() {
alert("An error had occured!");
}

});
});
});


在ie下测试正常，在ff中每次都是显示“An error had occured!”