hi, i have wrote a method that filters output with provided query and return it. when one Where excuted; it return correct output but when more than one Where excuted; output is null and Exception occured with message "Enumeration yielded no results". why? how i can fix it?

public IQueryable<SearchResult> PerformSearch(string query, int skip = 0, int take = 5)
    {
        if (!string.IsNullOrEmpty(query))
        {
            var queryList = query.Split('+').ToList();
            var results = GENERATERESULTS();
            string key;
            foreach (string _q in queryList)
            {
                if (_q.StartsWith("(") && _q.EndsWith(")"))
                {
                    key = _q.Replace("(", "").Replace(")", "");
                    results = results.Where(q => q.Title.Contains(key, StringComparison.CurrentCultureIgnoreCase));
                }
                else if (_q.StartsWith("\"") && _q.EndsWith("\""))
                {
                    key = _q.Replace("\"", "").Replace("\"", "");
                    results = results.Where(q => q.Title.Contains(key, StringComparison.CurrentCulture));
                }
                else if (_q.StartsWith("-(") && _q.EndsWith(")"))
                {
                    key = _q.Replace("-(", "").Replace(")", "");
                    results = results.Where(q=> !q.Title.Contains(key, StringComparison.CurrentCultureIgnoreCase));
                }
                else
                {
                    key = _q;
                    results = results.Where(q => q.Title.Contains(key, StringComparison.CurrentCulture));
                }
            }
            this._Count = results.Count();
            results = results.Skip(skip).Take(take);
            this._EndOn = DateTime.Now;
            this.ExecutionTime();
            return results;
        }
        else return null;
    }

thanks in advance ;)