Inserting a new entry to Datagrid (WPFToolkit) with Linq-Query doesn't work

Question

I am filtering the list of entries from my DataContext with a Linq-statement. After that I assign them to the the ItemsSource-property of the DataGrid available in the WPFToolkit. The available entries are being shown, but for some reason I am not able to insert a new row into the Datagrid. Surprisingly it all works fine if do not use the LINQ statement but assign the System.Data.Linq.Table object directly.

the linq query is as follows:

(from n in controller.DataContext.DictionaryEntries where (n.Dictionary == newDict) select n).ToList();

Any idea to resolve this is appreciated. L. Miller