In my case, it was just a silly "!" mark :( let params = [         "_id": data.id       ] as! [String:String] I removed "!" beside "as" because xCode was showing a warning.! Then I ran into this issue.. I wasted almost 2 hours deleting & installing xCode and derived data files.. Although the error was show-cased in Issues Pane.. I run through each error and finally got to this one.. When I added "!", the weird error was gone.! Hope this Helps.! Thanks.