Call to a member function fetch_assoc() on array

  $author_content = $db->query( "SELECT * FROM post WHERE author = '$id'" );
  while ( $author_content = $author_content->fetch_assoc() )
  {
    $content = '<a href="post.html?id=1"><img src="http://itd1.mycdn.me/image?id=869556971587&t=20&plc=WEB&tkn=*PzRyN3OlodQ660m0B4oIo4M3kKs" align="center">'.$author_content['name'].'</a>';
  }

Пишет:

[Sun Jun 02 10:18:09.317923 2019] [lsapi:error] [pid 1946075] [client 62.148.226.69:37464] [host history.kiselyov.ml] Backend fatal error: PHP Fatal error: Uncaught Error: Call to a member function fetch_assoc() on array in /var/www/user63337/data/www/history.kiselyov.ml/author.php:22\nStack trace:\n#0 {main}\n thrown in /var/www/user63337/data/www/history.kiselyov.ml/author.php on line 22\n