Return LINQ populated ViewModel from Business Logic Layer to Controller - MVC

I have a LINQ query that populates a view model of StaffHierarchySearchViewModel. I'm trying to populate StaffHierarchySearchViewModel in my Business Logic Layer (BLL) and return those results to my controller. When I attempt to return the results value in my method, I'm getting errors on the return results line. I'm getting errors of: Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<WebReportingToolDAL.Models.ViewModels.StaffHierarchySearchViewModel>' to 'WebReportingToolDAL.Models.ViewModels.StaffHierarchySearchViewModel'. An explicit conversion exists (are you missing a cast?)' I've tried other workarounds, but I still can't seem to get the return value to work.

namespace WebReportingToolBLL
{
 public class StaffHierarchySearch
  {
    private Collections_StatsEntities db = new Collections_StatsEntities();

    public StaffHierarchySearchViewModel StaffHierarchySearchViewModel(string SearchString)
    {
        IEnumerable<StaffHierarchySearchViewModel> results = from sa in db.SecurityAccesses
                     join ss in db.SecuritySystems on sa.System equals ss.SystemID
                     join s in db.Staffs on sa.EmpID equals s.EmpID
                     join u in db.Units on s.UnitID equals u.UnitID
                     join h in db.Hierarchies on sa.EmpID equals h.EmpID
                     join t in db.Staffs on h.TeamID equals t.EmpID
                     where sa.ID == SearchString
                     select new WebReportingToolDAL.Models.ViewModels.StaffHierarchySearchViewModel
                     {
                         ID = sa.ID,
                         SystemName = ss.SystemName,
                         LastName = s.LastName,
                         FirstName = s.FirstName,
                         UnitName = u.UnitName,
                         EffectiveStart = h.EffStart,
                         EffectiveEnd = h.EffEnd,
                         ManagerFirstName = t.FirstName,
                         ManagerLastName = t.LastName,
                         Search = SearchString
                     };

        return results;
    }

}

}

Going off of what @James Curran said, I updated the method return type to List<StaffHierarchySearchViewModel> as well as add return results.ToList(); It seems to be working as expected.

namespace WebReportingToolBLL
{
public class StaffHierarchySearch
{
    private Collections_StatsEntities db = new Collections_StatsEntities();

    public List<StaffHierarchySearchViewModel> StaffHierarchySearchViewModel(string SearchString)
    {
        IEnumerable<StaffHierarchySearchViewModel> results = from sa in db.SecurityAccesses
                     join ss in db.SecuritySystems on sa.System equals ss.SystemID
                     join s in db.Staffs on sa.EmpID equals s.EmpID
                     join u in db.Units on s.UnitID equals u.UnitID
                     join h in db.Hierarchies on sa.EmpID equals h.EmpID
                     join t in db.Staffs on h.TeamID equals t.EmpID
                     where sa.ID == SearchString
                     select new WebReportingToolDAL.Models.ViewModels.StaffHierarchySearchViewModel
                     {
                         ID = sa.ID,
                         SystemName = ss.SystemName,
                         LastName = s.LastName,
                         FirstName = s.FirstName,
                         UnitName = u.UnitName,
                         EffectiveStart = h.EffStart,
                         EffectiveEnd = h.EffEnd,
                         ManagerFirstName = t.FirstName,
                         ManagerLastName = t.LastName,
                         Search = SearchString
                     };

        return results.ToList();
    }

}

}