json_encode数组上返回空值

我正在尝试json_encode一个php数组，但是"选择"部分返回null。

Array ( [indexer] => 7 [section] => 1 [question] => What does the last paragraph indicate about an optimistic perspective? [answer] => a [choices] => There has never been a culture or religion that is genuinely optimistic`It is “a riddle of the universe”`No enlightened culture sees the world in a truly optimistic manner`Optimistic perspectives are only held by the weak )
{"indexer":"7","section":"1","question":"What does the last paragraph indicate about an optimistic perspective?","answerswer":"a","choices":null}

有什么想法吗？

编辑：完整代码：

$check = mysqli_query($con, "select * from questions where section = '$section' and indexer = '$question'");
$result = mysqli_fetch_array($check, MYSQLI_ASSOC);
print_r($result);
echo json_encode($result);