title: Cocoa Error 3840 when POST Request with AFNetworking 2
body: <p>I call this function and return Cocoa Error 3840 every time.
  I try to debug and fix it and it error when request rather than when parse the result</p>
  
  <hr>
  
  <p>I found this error in failure blocks when request.</p>
  
  <pre><code>Error Domain=NSCocoaErrorDomain Code=3840 "The operation couldn’t be completed. (Cocoa error 3840.)" (JSON text did not start with array or object and option to allow fragments not set.) UserInfo=0x109230960 {NSDebugDescription=JSON text did not start with array or object and option to allow fragments not et.}
  2013-11-01 12:09:30.925 MagicBox[87431:70b] The operation couldn’t be completed. (Cocoa error 3840.)
  </code></pre>
  
  <p>That's my code 
  Thanks in Advance.</p>
  
  <pre><code>- (void)loginWithUserName:(NSString *)userName
               Password:(NSString *)password
             orFacebook:(NSString *)facebookID
      withResponseBlock:(ResponseBlock)responseBlock {
  
  if (!userName &amp;&amp; !facebookID) {
      NSError *error = [NSError errorWithDomain:@"Missing Parameters"
                                           code:400
                                       userInfo:@{ NSLocalizedDescriptionKey : @"Username or FacebookID is required"}];
      responseBlock(error, nil);
  }
  
  NSDictionary *params;
  
  if (facebookID) {
      params = @{ @"fb_id": facebookID };
  } else {
      params = @{ @"username": userName,
                  @"password": password };
  }
  
  AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
  NSString *loginURL = [self requestWithPath:@"/api/login"];
  [manager POST:loginURL
     parameters:params
  constructingBodyWithBlock:nil
        success:^(AFHTTPRequestOperation *operation, id responseObject) {
            if (responseBlock) {
                responseBlock(nil, responseObject);
            }
        }
        failure:^(AFHTTPRequestOperation *operation, NSError *error) {
            if (responseBlock) {
                responseBlock(error, nil);
            }
        }];
  }
  </code></pre>