Login error tidak dapat masuk

saya bingung pada saat masuk ke form login , tiba tiba keluar error " Fatal error: Call to a member function fetch_assoc() on boolean in C:\xampp\htdocs\aplikasi_jadwal_mahasiswa\login_act.php on line 11"

bagaimana cara memperbaikinya??

<?php
   session_start();
   require_once("db_connect.php");

   $user = $_POST['user'];
   $pass = $_POST['pass'];

   $sql = "SELECT * FROM tb_user WHERE user= '$user'";

   $query = $db->query($sql);
   $hasil = $query->fetch_assoc();

   if($query->num_rows == 0) {
     echo "<div align='center'>Username Belum Terdaftar! <a href='index.php'>Back</a></div>";

   } else {
     if($pass <> $hasil['pass']) {
       echo "<div align='center'>Password salah! <a href='index.php'>Back</a></div>";

     } else {
       $_SESSION['user'] = $hasil['user'];
       header('location:table-jadwal.php');
     }
   }
?>s