That frustrates. I'm russian and often face problems of "article bureaucracy". I think, tenfef's translation should be marked as "Ooops, may be there must be an article", but not by removal of "heart".
I totally agree with Bladekona. For many of us, English is not our first language, and Duolingo should not be so strict when it comes to articles. After all, we are trying to learn French, not English.
"au" in front of "café" is the contracted article of "à-le", that is why it is a definite article.
therefore, if you translate "au café", you have to use article "the".
note: in English "add to" - in French "ajouter à"