Why c# Dictionary keys convert to array,show exception message "Value cannot be null. Parameter name: key"

private static Dictionary<string, int> _dics = new Dictionary<string, int>() ;

   public TestController()
   {
       if (_dics.Count == 0)
       {
           _dics.Add("test1", 1);
           _dics.Add("test2", 2);
           _dics.Add("test3", 3);
           _dics.Add("test4", 4);
       }
   }

  var removes = _dics.Select(s => s.Key).ToArray();
            foreach (var item in removes)
            {
              _dics.Remove(item);
            }