PDO Update query: PDOException: SQLSTATE[HY093]: Invalid parameter number – but the number seems to be correct [duplicate]

I’m confused: I’m getting this error for an invalid number of parameters but I think the number is correct. There must be something I’m missing and I need your help figuring out what.

This is the array I want to INSERT … ON DUPLICATE KEY UPDATE:

$payable_data = array(
    'projectID' => $_POST['projectID'],
    'date'      => $project_data['duedate'],
    'vendorID'  => $project_data['vendorID'],
    'totaltopay'=> ($project_data['subprice'] * $project_data['units']),
    'currency'  => $project_data['currency'],
    'paid'      => '0'
);

Six items total. Then I build the UPDATE part of the query:

$update = "";
foreach($payable_data as $payable_key => $payable_value) {
    $update .= " ".$payable_key." = ?,";
    $update_values[] = $payable_value;
}
$update = substr($update, 0, -1);

Here’s my query (I use a function to create the ? placeholders):

$payableQuery = $db->prepare("INSERT INTO ".PAYABLES." (".implode(',', array_keys($payable_data)).") VALUES (".$options->placeholders($payable_data).") ON DUPLICATE KEY UPDATE".$update."");

And here I execute the query:

$payableQuery->execute($update_values);

Here’s a screenshot of the error:

It’s relatively easy to count the number of parameters – six. What is wrong then?