HashTable could not be found

I am using System.Collections, and yet, when I try to use a HashTable, Unity complains about that:

error CS0246: The type or namespace name `HashTable' could not be found. Are you missing a using directive or an assembly reference?

My setup is Unity 4.1 on Windows 7 x64. Anyone knows how can I use HashTable? I don’t want to build my own just to avoid this…

The class name is Hashtable, not HashTable.

I, too, recommend using the Dictionary class.