It's definitely correct too. If Duolingo doesn't accept it, I believe it should be reported.
After н, both ы and и are possible.
When the stem (the part of the word to which endings are attached) ends in hard -н, the ending is ны: сло́н ‘elephant’ — слоны́ ‘elephants’.
When the stem ends in soft -нь, the ending is ни: конь ‘horse’ — ко́ни ‘horses’.