Many languages DO presume a verb of being in such a case. I did the same because of the presence of the period. Until now, it's been duolingo's habit to leave off a period in phrases that do not contain verbs. Ah, well!
That is exactly what threw me for a loop! I saw the period and assumed it was a complete sentence. Generally, DL phrases that are not a complete sentence do not have the period, and that's a clue. Lesson learned, I guess!