Set optionset value to null or empty? c#

Hi I am working on a plugin, but I am having a hard time finding a solution to set an optionset to null.

I've tried the following and get no result:

tempContact["new_test"] = new OptionSetValue(-1);
tempContact["new_test"] = null;
tempContact.Attributes.Add("new_test", null);
tempContact.Attributes.Add("new_test", new OptionSetValue(-1));

None of these work, does anyone know the correct syntax?