Error passing Dictionary to IDictionary parameter

[nickfake]

Why is there an error

Error 52 Argument 1: cannot convert from 'System.Collections.Generic.Dictionary>' to 'System.Collections.Generic.IDictionary>'

Dictionary<string, List<string>> tempResultIDList = new Dictionary<string,List<string>>();
test(tempResultIDList);

public bool test(IDictionary<string,IList<string>> a)
{
return true;
}