Trouble cooking

Trying to cook on Linux, but I get this error:

TempStorage.cs(493,16): error CS0246: The type or namespace name `HashSet’ could not be found. Are you missing a using directive or an assembly reference?

I’m thinking there’s some .dll’s missing from the mono stuff in the DotNet directory. (?) I’m not all that familiar with mono, so I’m not sure how to fix this or what might be missing.