Mysterious disappearing blocks

I'm having trouble with my custom blocks. I place them in the world and then I quit the game and when I reopen the game they're gone! I've narrowed the problem down to my blocks tile entity, it seems when I comment out the nbt write / read functions it works fine. I'm not sure what the problem is

Here's my nbt code

public void writeToNBT(NBTTagCompound nbt) {
	super.writeToNBT(nbt);

	nbt.setIntArray("group", this.group);
	nbt.setInteger("speed", this.speed);
	nbt.setInteger("ticksTillCheck", this.ticksTillCheck);
}

public void readFromNBT(NBTTagCompound nbt) {
	super.readFromNBT(nbt);

	this.group = nbt.getIntArray("group");
	this.speed = nbt.getInteger("speed");
	this.ticksTillCheck = nbt.getInteger("ticksTillCheck");
}