If your mob has an error in it when you are trying to set what it drops, you may have set it to drop a block! You need some extra code to drop a block. The code is:

Item.getItemFromBlock()

This function is used to get the Item form of a Block if it has one. It would be used like this:

this.dropItem(Item.getItemFromBlock(Blocks.obsidian),64);
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