System.InvalidOperationException: Collection was modified; enumeration operation may not execute..

foreach(var book in mCart)
{
   if(book.Value > 0)
      mCart[book.Key]--;
}

Dictionary<string, int> test = new Dictionary<string, int>();
            test.Add("eins", 0);
            test.Add("zwei", 3);
            test.Add("drei", 5);

            foreach (string tmp in new LinkedList<string>(test.Keys))
            {
                if (test[tmp] > 0)
                {
                    test[tmp]--;
                }
            }

            foreach (KeyValuePair<string, int> t in test)
            {
                Console.WriteLine(t.Key+"  "+t.Value);
            }
            Console.ReadKey();