PHP, MySQL:não consigo explicar esse erro de índice indefinido

// Select all bookings
$sql = "SELECT * FROM booking";

$result = mysqli_query($con,$sql);

echo "<p>";

while ($row = mysqli_fetch_array($result, MYSQLI_ASSOC)) {

    print_r( $row );  // debug code

    // echo $row['custName'] . " - " . $row['roomNb'] . " - " . $row['date'];
    echo $row['custName'];
}