Fatal error: Call to a member function fetch_assoc() on boolean

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "database";
$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT * FROM clients GROUP BY campaign";
$result = $conn->query($sql);
while($row = $result->fetch_assoc()) {
    $campaign = $row['campaign'];
    echo $campaign;
}