PDO inserting into database not working

when I type in the information that will be inserted into the database I receive this error: Fatal error: Call to a member function execute() on a non-object. This is my code so far.

    <?php
    $database_file = 'sqlite:inventory.sqlite';
    $mysql = new PDO($database_file);

    $new_name = $_POST['Name_Item'];
    $new_amount = $_POST['Amount_Item'];
    $new_detail = $_POST['Description'];


  $query =$mysql->prepare("INSERT INTO Items (id, Name, Amount, Detail)
  VALUES (''," . $new_name . "," . $new_amount . ",". $new_detail .")");
  $query->execute();
  echo "testing";
  echo "<TABLE>";
  echo "<tr>";
  echo "<td>id</td>";
  echo "<td>Name</td>";
  echo "<td>Amount</td>";
  echo "<td>Detail</td>";
  echo"</tr>";
  while ($row = $query->fetch()) {
      //print_r($row);
      echo "<tr>";
      echo "<td>$row[id]</td>";
      echo "<td>$row[Name]</td>";
   }
 ?>