无法根据用法推断OrderBy类型的问题

我正在学习一门关于在.net中构建API的复数视觉课程，我似乎遇到了所提供代码的问题。我有一个类，它应该根据提供的查询参数对给定的集合进行排序。这是代码：

public static class IQueryableExtensions
{
    public static IQueryable<T> ApplySort<T>(this IQueryable<T> source, string sort)
    {
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }
        if (sort == null)
        {
            return source;
        }
        // split the sort string
        var lstSort = sort.Split(',');
        // run through the sorting options and create a sort expression string from them
        string completeSortExpression = "";
        foreach (var sortOption in lstSort)
        {
            // if the sort option starts with "-", we order
            // descending, otherwise ascending
            if (sortOption.StartsWith("-"))
            {
                completeSortExpression = completeSortExpression + sortOption.Remove(0, 1) + " descending,";
            }
            else
            {
                completeSortExpression = completeSortExpression + sortOption + ",";
            }
        }
        if (!string.IsNullOrWhiteSpace(completeSortExpression))
        {
            source = source.OrderBy(completeSortExpression.Remove(completeSortExpression.Count() - 1));
        }
        return source;
    }
}

问题出在线路上：

source = source.OrderBy(completeSortExpression.Remove(completeSortExpression.Count() - 1));

由于某种原因，OrderBy抛出错误：the type for method OrderBy cannot be inferred from the usage. Try specifying the type arguments explicitly.

using System.Linq.Dynamic;