In one of nthuleen's exercise sheets, it is given that (question 1.4, see the answer sheet)
Herbert ist ein guter Freund (m) von mir.
However, "Freund" is a direct object in this sentence, hence it is an accusative case. According to the declension table of Laura, it should have been
Herbert ist einen guten Freund (m) von mir.
But the answer key declines the adjective as if it is a nominative case. Am I missing something in here or is the answer sheet wrong?