CS0012 The type 'MvcHtmlString' is defined in an assembly that is not referenced

@Html.PagedListPager(Model, page => Url.Action("Index",
                                               new { 
                                                        page, 
                                                        sortOrder = ViewBag.CurrentSort, 
                                                        currentFilter = ViewBag.CurrentFilter 
                                                   }))

This throws an error:

CS0012 The type 'MvcHtmlString' is defined in an assembly that is not referenced

It happened once I tried to add pagination.

@Html.PagedListPager this part is the problem I think