Tileentity not saving inventory

I am getting this error message when trying to write NBT to save the inventory, if anyone could tell me what I am doing wrong I would appreciate it!

[02:15:01] [Server thread/ERROR] [FML]: A TileEntity type ronaldi2001.MoreItems.blocks.tileentity.TileEntityUpgrader has throw an exception trying to write state. It will not persist. Report this to the mod author
java.lang.RuntimeException: class ronaldi2001.MoreItems.blocks.tileentity.TileEntityUpgrader is missing a mapping! This is a bug!
	at net.minecraft.tileentity.TileEntity.writeInternal(TileEntity.java:89) ~[TileEntity.class:?]
	at net.minecraft.tileentity.TileEntity.writeToNBT(TileEntity.java:80) ~[TileEntity.class:?]
	at ronaldi2001.MoreItems.blocks.tileentity.TileEntityUpgrader.writeToNBT(TileEntityUpgrader.java:124) ~[TileEntityUpgrader.class:?]
	at net.minecraft.world.chunk.storage.AnvilChunkLoader.writeChunkToNBT(AnvilChunkLoader.java:416) [AnvilChunkLoader.class:?]
	at net.minecraft.world.chunk.storage.AnvilChunkLoader.saveChunk(AnvilChunkLoader.java:191) [AnvilChunkLoader.class:?]
	at net.minecraft.world.gen.ChunkProviderServer.saveChunkData(ChunkProviderServer.java:214) [ChunkProviderServer.class:?]
	at net.minecraft.world.gen.ChunkProviderServer.saveChunks(ChunkProviderServer.java:242) [ChunkProviderServer.class:?]
	at net.minecraft.world.WorldServer.saveAllChunks(WorldServer.java:1062) [WorldServer.class:?]
	at net.minecraft.server.MinecraftServer.saveAllWorlds(MinecraftServer.java:467) [MinecraftServer.class:?]
	at net.minecraft.server.integrated.IntegratedServer.saveAllWorlds(IntegratedServer.java:274) [IntegratedServer.class:?]
	at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:177) [IntegratedServer.class:?]
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:589) [MinecraftServer.class:?]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_161]

This is what is in my tile entity class

	@Override
	public void readFromNBT(NBTTagCompound compound)
	{
		super.readFromNBT(compound);
		this.inventory = NonNullList.<ItemStack>withSize(this.getSizeInventory(), ItemStack.EMPTY);
		ItemStackHelper.loadAllItems(compound, inventory);
		this.burnTime = compound.getInteger("BurnTime");
		this.cookTime = compound.getInteger("CookTime");
		this.totalCookTime = compound.getInteger("CookTimeTotal");
		this.currentBurnTime = getItemBurnTime((ItemStack)this.inventory.get(2));
		if(compound.hasKey("CustomName", 8)) this.customName = compound.getString("CustomName");
	}
	
	@Override
	public NBTTagCompound writeToNBT(NBTTagCompound compound) 
	{
		super.writeToNBT(compound);
		compound.setInteger("BurnTime", (short)this.burnTime);
		compound.setInteger("CookTime", (short)this.cookTime);
		compound.setInteger("CookTimeTotal", (short)this.totalCookTime);
		ItemStackHelper.saveAllItems(compound, inventory);
		if(compound.hasKey("CustomName", 8)) compound.setString("CustomName", this.customName);
		
		return compound;
	}