naivan

1. naive; gullible
   Bio sam naivan i poverovao sam mu. — I was naive and believed him.
   To je veoma naivno pitanje. — That's a very naive question.
2. colloq. (usually with negation) harmless; trivial
   Povreda nije naivna, pauziraće mesec dana. — The injury is no joke, he'll be out for a month.
   Na prvi pogled je delovalo naivno. — At first glance it looked harmless.

"Nije naivno" usually means "it's serious", not "it isn't naive", though the literal meaning is also possible in a suitable context.