It's because the game undergoes an integer overflow as the code in the lecture file has not yet been added to the video series. The paid file includes the full lecture. I would recommend budgeting for it first, and upon buying it to use it as a back up file in case any modifications to your file goes wrong, for example adding a new feature. Name it something like "OriginalBackupSourceBattleSystem". I'd rather have us learning than get frustrated waiting for lecture videos.