[1.7.10]Writing itemstack to NBT

So I have an entity which has a saved itemstack as a value. I want to save it between world unload and load event but it doesn't seem to be working. Currently this is what my writeEntityToNBT and readEntityFromNBT are:

writeEntityToNBT:

public void writeEntityToNBT(NBTTagCompound tag){
	super.writeEntityToNBT(tag);
	System.out.println("Written to NBT");
	drop.writeToNBT(tag);
	System.out.println(tag);
}

readEntityFromNBT:

public void readEntityFromNBT(NBTTagCompound tag){
	super.readEntityFromNBT(tag);
	System.out.println("Read from NBT");
}

when the write method is called, everything works. However when the read method seems to never be called once i add the "drop.writetoNBT(tag)" line to the write method. I noticed the console prints

[16:02:08] [server thread/WARN]: Skipping Entity with id 4097s

which is the id of the itemstack that was saved in the write method. Any ideas why this happens?