core data - Swift3 CoreData with Extension -

i'm working on core data extensions, , having errors below.

terminating app due uncaught exception 'nsinvalidargumentexception', reason: 'executefetchrequest:error: not valid nsfetchrequest.'

it's working on host app still, crashing on extensions same codes.

debugger stopped when executed managedobjectcontext.fetch(request)

func getdatastring() -> string {      var result = ""      let entitydescription = nsentitydescription.entity(forentityname: "datastring", in: managedobjectcontext)!     let request : nsfetchrequest<nsfetchrequestresult> = datastring.fetchrequest()     request.entity = entitydescription      {         let results = try managedobjectcontext.fetch(request)         printdebug(message: "\tdatastring count : \(results.count)")          if results.count > 0 {             let row = results[0] as! nsmanagedobject              result = row.value(forkey: "data") as! string         }     } catch let error {         printdebug(message: "error : getdatastring fetch")         seterrorlog(errortext: "\(error)")     }      return result } 

this crashes though written try-catch. please help.

the generic type of nsfetchrequest has datastring:

let request: nsfetchrequest<datastring> = datastring.fetchrequest()  managedobjectcontext.perform {     {         let results = request.execute()     } catch let error {         ...     } }