CloudKit错误：无法修改某些记录

Question

所以我创建了这个函数来将资源上传到CloudKit。

func uploadAsset(recordType: String, fileURL: URL, reference: String) {
    let record = CKRecord(recordType: recordType)
    record["file"] = CKAsset(fileURL: fileURL)

    let modifyRecordsOperation = CKModifyRecordsOperation(
        recordsToSave: [record],
        recordIDsToDelete: nil)

    modifyRecordsOperation.timeoutIntervalForRequest = 10
    modifyRecordsOperation.timeoutIntervalForResource = 10

    modifyRecordsOperation.modifyRecordsCompletionBlock =
        { records, recordIDs, error in
             print("DEBUG: smodifyRecordsCompletionBlock reached")
            if let err = error {
                print("DEBUG: uhoh error " + err.localizedDescription)
            } else {
                print("DEBUG: success")
            }
    }

    modifyRecordsOperation.perRecordProgressBlock = {
        self.updateProgressView(progress: Float($1))
        print( "DEBUG: progress = ", $1 )
    }

    modifyRecordsOperation.completionBlock = {
        self.hideProgressView(hide: true)
        print("DEBUG: Ding, completion block hit!")
    }

    if (!Reachability.isInternetAvailable()) {
        print("DEBUG: uhoh error no internet connection")
    } else {
        CKContainer(identifier: "iCloud.com.example-container").publicCloudDatabase.add(modifyRecordsOperation)

        print("DEBUG: should have started cloud upload")
    }
}

现在这对我自己来说非常好，但对我的客户来说却失败了。但错误信息并不是最有见地的，它说：

DEBUG: should have started cloud upload
file saved
DEBUG: smodifyRecordsCompletionBlock reached

我已经仔细检查过，我的客户在他的测试设备上登录了iCloud。我在这里缺少什么？