jQuery Grid Plugin - jqGrid

How to disable leaf node icon? I tried

treeIcons: { leaf: '' },

treeIcons: { leaf: null },

treeIcons: { leaf: 'nonexistingstyle' },

but default icon appears in this case.

            grid.jqGrid({
                url: '<%= ResolveUrl("~/Store/GridData")%>',
                datatype: "json",
                mtype: "POST",
                height: "auto",
                loadui: "disable",
                treeGridModel: "adjacency",
                colModel: [
                        { name: "id", width: 1, hidden: true, key: true },
                        { name: "menu" },
                        {name: "url", width: 1, hidden: true }
                    ],
                autowidth: true,
                treeGrid: true,
                ExpandColumn: "menu",
                rowNum: 200,
                ExpandColClick: true,
                treeIcons: { leaf: null },  // todo: why this does not remove icon

                onSelectRow: function (rowid) {
                    var treedata = grid.jqGrid('getRowData', rowid);
                    window.location = '<%= ResolveUrl("~/")%>' + treedata.url;
                }

            }
                    );
            grid.parents("div.ui-jqgrid-view").children("div.ui-jqgrid-hdiv").hide();