I'm getting an error when trying to run this code in Unity 2018.3 : The detailed error message says "The value for key test_key was not valid JSON."
PlayFabClientAPI.UpdateUserData(new UpdateUserDataRequest() { Data = new Dictionary<string, string>() { {"key", "value"} }, Permission = UserDataPermission.Public }, result => Debug.Log("Successfully updated user data"), error => { Debug.Log("UpdateUserData error"); });