Solved Writing BasePlayer to file?

I'm trying to save Dictonary<BasePlayer, Vector3> to file using Write/ReadObject. But there is a error (couldn't cast from string to BasePlayer). It seems like I can't save BasePlayer object to file using neither these functions nor Config (which also uses them). So, how can I save it?

UPD: The thread is closed. I just changed indexes to ulong IDs instead of the whole player object.