Parsing JSON Swift 3 with error Code=3840 - xcode

Question

I have this error.
Error Domain=NSCocoaErrorDomain Code=3840 "Garbage at end." UserInfo={NSDebugDescription=Garbage at end.}
Here is my swift code.
let request = NSMutableURLRequest(url: NSURL(string: "http://example.org/file.php")! as URL)
request.httpMethod = "POST"
let task = URLSession.shared.dataTask(with: request as URLRequest) {
old_data, response, error in
//Error Checking
if error != nil {
print("error=\(error)")
return
}
print("response = \(response)")
//Response string built up
let responseString = NSString(data: old_data!, encoding: String.Encoding.utf8.rawValue)
print("responseString = \(responseString)")
//Manipulating the JSON data
var names = [String]()
do {
if let new_data = old_data,
let json = try JSONSerialization.jsonObject(with: new_data) as? [String: Any],
let buildings = json["buildings"] as? [[String: Any]] {
for building in buildings {
if let name = building["BuildingName"] as? String {
names.append(name)
}
}
}
} catch {
print(error)
}
print("Names: \(names)")
}
task.resume()
The error is happening in my 'do' statement where 'let buildings = json["buildings"]'
And below is what I get when I print the response string.
responseString = Optional({"buildings":[{"BuildingID":"2","BuildingName":"School of Informatics and Comp","Info":"School of higher learning.","Latitude":"39.172085","Longitude":"-86.522908"}]}Successfully Retrieved. SELECT * FROM Buildings;)
Any questions or advice is appreciated