C# Dictionary include list as value, when list change, dictionary change

Question

I Created a Dictionary like this:

Dictionary<string, List<string>> MS = new Dictionary<string, List<string>>();

and Created a list like this:

List<string> Sig = new List<string>();

i need to read some strings by Sig, and add Sig to MS- the dictionary. since there are different strings, after add Sig to MS, i clear Sig in order to read new strings,

 MS.Add(Mess1, Sig);
 Sig.Clear();

but when i clear Sig, the MS cleared too. so what can i do? thanks a lot!