It's a strange quirk of Modern Hebrew (and it also occurs a few times in the Bible, so you can't just say that modern speakers are ruining the language). It's doesn't make any sense from a logical point of view, but we always use את with יש and אין if the thing possessed is definite.
אין לי את זה
יש לי את זה
Because although it's a literal translation of the Hebrew, it's poor English grammar. Perhaps 500 years ago it might have been common to say "I have not this..." but these days, unless you're trying to sound poetical, it would be usual to say "I haven't got this appliance" or "I don't have..."