CognitoSyncDemo: Failed to list records in dataset

I downloaded Amazon's samples: CognitoSyncDemo http://ift.tt/1CaZ2nJ and tried to run it in unauthenticated mode. I followed Amazon's guide: created AWS account; created Identities Pool in Cognito; added roles in IAM.

But when I ran the sample app, populate some data into dataset and tried to sync them, it popup that : Failed due to Failed to list records in dataset: XXX(XXX is my dataset name).

The role policy I created for unauthenticate is as follows:

{
    "Version": "2012-10-17",
    "Statement": [{
        "Action": [
            "mobileanalytics:PutEvents",
            "cognito-sync:*",
            "sns:*"
        ],
        "Effect": "Allow",
        "Resource": [
            "*"
        ]
    }]
}

I am not sure if it was the role's problem.

Does anyone had such problem as well when you ran the CognitoSyncDemo provided by Amazon?