jsonp syntax error unexpected token - jsonp

Question

if I use dataType: json I don't get an error in the response/
If I use dataType: jsonp I get the following error:
syntax error unexpected token
This is my jquery code
function testJsonp()
{
$.ajax({
type: "POST",
url: "http://localhost/FT8Services/FTService.asmx/testjsonp",
data: "{firstname: '" + encodeURIComponent("Stack") + "', lastname: '" + encodeURIComponent("Overflow") + "'}",
contentType: "application/json; charset=utf-8",
dataType: "jsonp",
success: function (result) {
var loginfo = eval(result.d);
alert(loginfo[0]);
if(loginfo.length > 1)
{
alert("Sucess");
}
else
{
alert("no access");
}
},
error: function(xhr, status, error) {
alert(error);
}
})
}
and this is my asp.net web method
<WebMethod()> _
Public Function testjsonp(ByVal firstname As String, ByVal lastname As String) As String
Dim rdatas As New List(Of String)
Dim fullname As String = firstname & lastname
For i As Integer = 1 To 5
rdatas.Add(fullname & " " & i.ToString)
Next
Dim js As New JavaScriptSerializer
Dim strJSON As String = js.Serialize(rdatas.ToArray)
Return strJSON
End Function