SPListItemCollection to Dictionary

Rather than repeatedly query a SharePoint list for information I need, I wanted to create a dictionary from an SPListItemCollection (where the dictionary key is the “Title” value of each SPListItem, and the value is the SPListItem itself). I tried this:

SPListItemCollection documentCodesListItems = documentCodesList.GetItems(query);

Dictionary<string, SPListItem> dictionary = (from SPListItem i in documentCodesListItems select new { Key = i.Title, Value = (SPListItem)i }).ToDictionary(
                a => a.Key,
                a => a.Value);

But I get "Value does not fall within the expected range." exception. Can anybody see my mistake, or should I do this differently?