idioms - "Add-in salt to injury"?

I've never seen "Add-in salt to injury" but I know "Add insult to injury" exists. I had a grammar exercise that asked for the most suitable idiom or proverb for expressing:

To make something bad become worse

So I wrote "Add insult to injury". But the scheme says the correct answer is "Add-in salt to injury". So, which one is correct?

I've googled for "Add-in salt to injury" but the only result was the "add insult" version.

Answer

The test's answer “add-in salt to injury” is a mishearing and combination of two idioms: “add insult to injury,” (to mock, ridicule, or worsen something that is already bad) and “rub salt in a wound” (to worsen physical or emotional pain.)

The misheard phrase, and its variants:

add salt to injury
adding salt to injury

is known as an eggcorn, which has replaced the older term mondegreen.

Although the term mondegreen has been used for misheard phrases not from songs and poems, eggcorn, which originated in a 2003 Language Log post, has been advanced as a broader term for misheard words or phrases that retain their original meanings. So, for example, doggy-dog world is an eggcorn because it’s used in roughly the same way as the original phrase, dog-eat-dog world. Grammarist

In actual fact, the eggcorn has found its way in Google Books, 23 results for “add salt to injury”, and 33 results for “adding salt to injury” While the original and correct idiom, “add insult to injury”, has over 47,000 hits

“Add(-in) salt” sounds very similar to “add insult”, while “injury” is a synonym of “wound”, so it's not surprising people have mixed the two idioms together.