ทำไมถึง alert เป็น [object HTMLInputElement] หรอครับ

if($_SESSION['ID'] == "1") { echo "
                    <div class='btn-group'>
                       <button onclick='keepValueById(idKeeper,".$row['eID'].")' class='btn btn-flat' id='btnn' data-toggle='modal' href='#demoModal' name='btn'><span class='awe-remove'></span></button>
                    </div>";}

function keepValueById(idKeeper,value) 
{  
    alert(idKeeper);   // [object HTMLInputElement]
    document.getElementById(idKeeper).value = value;
    
 return;   
}