But wait, didn't professions come without an article?
Depending on whether you use the article or not, you can interpret two meanings:
Ok, but while there is a difference in meaning in the Swedish sentences , they both translate to English as "He is a thief ", so both Swedish sentences should be accepted.
Added it now. As you say, both are perfectly possible translations without context.
Edit: Immediately got an email from Duolingo about it being accepted. Apparently I was one of the people who'd reported it.
Tack så mycket!
"Han är tjuv." still wrong. So I report it. 2016/11/12
Definitely an appropriate sentence for the Politics lesson.