php - mysqli prepared statement update query fails when id is cast as integer but not as string

In the below code, I am attempting to take three form variables ($nps,$sch,$joint) and an id ($weld_id) and insert them into an UPDATE query. The problem is that I get $stmt->error "No data supplied for parameters in prepared statement."

When I cast the id of the row as "i". The weird thing is that the statement will execute error free if I put single quotes around the last question mark and cast as 's', however, the actual database row will not update. The function on row 3 does NOT use prepared statements to select the current values for this row in the database.

I have var_dumped all variables, copied them and successfuly run the query on MySQL workbench. I am out of ideas, please help.

$weld = mysqli_real_escape_string($db,$_POST['id']);
$weld = single_weld_query($db,$weld);
if(isset($_POST['edit_weld_parameters'])){
    // Query to update 3 parameters on database entry where id = N
    $stmt       = $db->prepare("UPDATE `welds` SET `size` = '?' , `sch` = '?' , `joint` = '?' WHERE `id` = ?;");
    $stmt->bind_param("sssi", $nps, $sch, $joint, $weld_id);

    $nps        = isset($_POST['size'])?    mysqli_real_escape_string($db,$_POST['size']):          $weld['size'];
    $sch        = isset($_POST['sch'])?     mysqli_real_escape_string($db,$_POST['sch']):           $weld['sch'];
    $joint  = isset($_POST['joint'])?   mysqli_real_escape_string($db,$_POST['joint']):         $weld['joint'];
    $nps        = (strlen($nps) and in_array($nps,$pipe_obj->sizes))?                       $nps:   $weld['size'];
    $sch        = (strlen($sch) and in_array($sch,$pipe_obj->schedules))?                   $sch:   $weld['sch'];
    $joint  = (strlen($joint) and in_array(strtoupper($joint),$pipe_obj->joint_types))? $joint: $weld['joint'];
    $weld_id    = $weld['id'];
    if($stmt->execute()){
        echo $weld['weld_number'].' parameters edited.';
    }else{

        echo $stmt->error;
    }
}else{
    echo 'ERROR: Form failure.';
}