Alamofire4 jsonSerializationFailed in response - ios

Question

I use Alamofire4 and I have a jsonSerializationFailed error in responseJson.
It seems that the response is not well formatted Json :
AlmF Failure Reason: jsonSerializationFailed(Error Domain=NSCocoaErrorDomain
Code=3840 "Invalid value around character 0." UserInfo=
{NSDebugDescription=Invalid value around character 0.})
AlmF Underlying error: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "Invalid value around character 0." UserInfo={NSDebugDescription=Invalid value around character 0.})
Here is the code displaying there error:
case .responseSerializationFailed(let reason):
print("AlmF Response serialization failed: \(error.localizedDescription)")
print("AlmF Failure Reason: \(reason)")
}
Here is the result of debugPrint(response) for
.responseJSON { response in:
=>
[Request]: http://myserver.com/Images.zip
[Response]: <NSHTTPURLResponse: 0x60000003cde0> { URL: http://myserver.com.com/Images.zip } { status code: 200, headers {
"Accept-Ranges" = bytes;
Connection = close;
"Content-Length" = 117127;
"Content-Type" = "application/zip";
Date = "Tue, 01 Nov 2016 21:28:23 GMT";
Etag = "\"701a02b-1c987-534eb9a863980\"";
"Last-Modified" = "Fri, 10 Jun 2016 12:23:18 GMT";
Server = Apache;
"X-Powered-By" = PleskLin;
Where should I format the Json, I thought Alamofire do that
Edit :
Before my code with Swift2 was:
.response { _, _, _, error in
if let error = error {
SpeedLog.print("DownloadVC Failed with error: \(error)")
} else {
SpeedLog.print("DownloadVC zip downloaded")
}