I write "The boys' hats are big"
Duo marks this as correct but displays the warning:
"You have a typo. The boys hats are big."
e.g. it seems to think that there should be no possessive apostrophe? What's going on here? (Also, because it's marked correct, there is no Report "My answer was correct" option, so I don't know how to feed this back to Duo except here).