Invalid Cast exception with hashtable

I have the following code.  expCollection is a hashtable that contains a collection of classes of type TRI.FarmSelect.Expense:

private void getBudgetItemPrices(Hashtable expCollection, int budgetID, int regionCode)
{
      /* Loop through expenses collection and for each expense find prices/costs and assign */
      foreach(TRI.FarmSelect.Expense expense in expCollection)
      {
            if (DB.getBudgetItemCost(Response, budgetID, regionCode, expense))
            {
                  /* do stuff... */
            }
      }
}

I get a runtime error "Specified cast is not valid" in the foreach statement.  I am positive that the expCollection contains classes of the Expense type.  Why would it give this runtime error? Thanks.