Really, DuoLingo?

I like DuoLingo. It’s a fun way to practice a language, and it seems to be well designed. I detect the operation of some well-implemented algorithms that adapt to your mistakes by giving you extra practice in things that you’ve messed up, for example.

However, every now and then, the program does something odd. Until now, I never felt like complaining about these glitches, which were more amusing than anything else. But today, while getting in a little Spanish practice, DuoLingo tossed me a softball. I’m confident that my answer was not wrong, but DuoLingo thought otherwise. Check out the figure here to see what happened.

I was miffed, because this unjust and unfair judgement broke a respectable streak of right answers. What do you think? Does DuoLingo have a leg to stand on here? I showed this to a native Spanish speaker who agrees that I was in fact not wrong.

Why, DuoLingo? Why so mean?