函数fetch_object（）在fetch循环中使用in时返回false

Question

当我在fetch while循环之外执行retrieveSubcategory（）时，它可以工作，但是从它内部返回false。

private function retrieveSubcategory($subcategory_ID){
    global $db;
    Subcategory = $db->query("SELECT subcategory_NL  FROM wyby_subcategory WHERE subcategory_ID=$subcategory_ID");
    echo '<pre>' , var_dump($Subcategory) ,'</pre>'; //returns false Why?
    $Subcategory = $Subcategory->fetch_object();
    return $Subcategory->subcategory_NL;
}

function RetrieveAllSubcat($CompID)
{
    $cats ="";
    global $database;

    $query = "SELECT DISTINCT(subcategory_ID) AS subcategory_ID FROM wyby_product WHERE company_ID=? ORDER BY subcategory_ID ASC";
    $ResultQuery = $db->prepare($queryString);
    $ResultQuery->bind_param('s',$Param);
    $ResultQuery->execute();
    $ProductQuery->bind_result($subcategory_ID);

    while($ProductQuery->fetch()){
        $cats .= " ". $this->retrieveSubcategory($subcategory_ID) .",";
    }
    echo $cats;
}