error using IL2CPP on build

we are using 6.3.1p1 version. when building on iOS with Mono, there isn't any problem, but when using IL2CPP, an error occurs as below while playing client on iPhone.

============ NullReferenceException: A null value was found where an object instance was required. at Network.Home+c__Iterator33.Reset () [0x00000] in :0 at BattleSelectManager.setStoryRounds (Int64 mstMissionStoryStageId) [0x00000] in :0 at BattleSelectManager.Start () [0x00000] in :0 at Replacements.RemotingServices.CreateClientProxy (System.Type objectType, System.String url, System.Object[] activationAttributes) [0x00000] in :0 Replacements.RemotingServices:CreateClientProxy(Type, String, Object[]) ==============

why my data turned into NULL when building IL2CPP?? I have no idea..