JSON conversion is getting failed - ios

Question

Please review my code am badly stuck on this issue.
JSON conversion is not happening & it is going into catch block & printing following error.
Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.}
I've tried everything which is suggested here on StackOverflow but no luck.
I've trimmed my code for better understanding.
import Foundation
class Server
{
class func convertStringToDictionary(_ data: Data) -> [String:Any]?
{
do
{
let convertedDict = try JSONSerialization.jsonObject(with: data, options: []) as? [String:Any]
return convertedDict
}
catch let error as NSError
{
print(error)
}
return nil
}
class func registerUser( userInfo: String)
{
let url = URL(string: "http://132.148.18.11/missedprayers/welcome/register")!
var request = URLRequest(url: url)
request.httpMethod = "POST"
let postString = "request=" + userInfo
request.httpBody = postString.data(using: .utf8)
//----------------------------------------
let task = URLSession.shared.dataTask(with: request)
{
data, response, error in
guard let data = data, error == nil
else
{
print("error=\(error)")
return
}
if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200
{
print("statusCode should be 200, but is \(httpStatus.statusCode)")
print("response = \(response)")
}
//--------------
let finalData = Server.convertStringToDictionary(data)
print(finalData)
}
task.resume()
}
func submitBtnTapped(_ sender: AnyObject)
{
let userInfoDict = [
"name":"Maaz Patel",
"phoneNum":"[phone]",
"email":"maaz#gmail.com",
"city":"pune",
"country":"India",
"dobEnglish":"11-02-1992",
"app":"Dalail",
"aqeeda":"Sufi",
"gender":"male",
"MCCycle":""
]
//-------------------
do
{
let jsonData = try JSONSerialization.data(withJSONObject: userInfoDict, options: [] )
let jsonStr = String.init(data: jsonData, encoding: String.Encoding.utf8)
Server.registerUser(userInfo: jsonStr!)
}
catch let error as NSError
{
print(error)
}
}
}
Consider "SubmitBtnTapped" function is getting called from somewhere.
When am trying on Postman it's working also on Android same web service is working fine.