jQuery Grid Plugin - jqGrid

Hi, and first of all thanks a lot for your wonderfull work !!

I want to use the sortable option of jqGrid ( rows sorting), everything seems to work fine except the 'placeholder' option : 

specify a classname in this option, even when using forcePlaceholderSize does nothing !

Did I miss something ?

Herre is the code :

	$("#jqGridArticles").jqGrid('sortableRows',{
		placeholder:"ui-state-highlight",
		forcePlaceholderSize: true ,
		distance:15, 
		forceHelperSize: true,
		opacity: 0.6,
		tolerance: 'pointer',
		cursor: 'crosshair',
		start: function(e,u){ // réinitialisation du résultat précédent
			$("#topMessage").hide().html();
		},
		stop : function (event, ui) {
			var newOrder = $("#jqGridArticles").getRowData();  
			$.ajax({
				url:'../ajax.php',
				data:{action:"sortArticles",newOrder:newOrder,id_origine:origine},
				success:function(reponse){
					reponse=JSON.parse(reponse);
					//console.log(reponse.statut);
					$("#topMessage").removeClass;
					$("#topMessage").addClass(reponse.statut).html(reponse.message).show();
					}
			});
		}
	}).disableSelection();

Thanks