doesnt match the test

I got this code here:

https://pastebin.com/jaD4tSZe

and I wrote the test

https://pastebin.com/ynqfdBmF

But the test says

>   REQUIRE( pluralize({"car", "dog", "cot"}) == make_vec({"cars", "dogs", "cots"}) )

with expansion:
  { "car", "dog", "cot" }
  ==
  { "cars", "dogs", "cots" }

Can someone help me find the mistake, please?