Inconsistent accessibility: base class

Trying to derive a class from KeyedCollection, I keep getting the following error:

Inconsistent accessibility: base class 'System.Collections.ObjectModel.KeyedCollection<string,Dell.Global.PartsClass.DBObjects.InvThreshold>' is less accessible than class 'Dell.Global.PartsClass.DBObjects.InvThresholdList'

Here's the code, it's pretty much straight from the sample in the KeyedCollection documentation:

using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

using System.Text;

namespace Dell.Global.PartsClass.DBObjects

public class InvThresholdList : KeyedCollection<string, InvThreshold>

{

public InvThresholdList() : base()

{

}

protected override string GetKeyForItem(InvThreshold objItem)

{

return objItem.AreaName + ":" + objItem.ClassificationName;

}

}