Converting TimeSpan to milliseconds in linq query

I'm trying to add a TimeSpan to a DateTime in a linq query using entity framework. I've got the following code:

EDIT

TimeSlotsis an ICollection<TimeSlot>

TimeSlot has the properties DateTime Start and TimeSpan Duration

var linq = from s in schedulesBase
           join c in channelBase on s.ChannelId equals c.ChannelId
           select new
           {
                ...
                TimeSlots = s.TimeSlots,
                ...
           };

var timeslots = linq.SelectMany(t => t.TimeSlots);

return from s in timeslots
       select new Resources.Event
       {
            ...
            end = DbFunctions.AddSeconds((DateTime?)s.Start, (Int32?)s.Duration.Seconds)
            ...
       };

It compiles fine but doesn't add any time from s.Duration.Seconds to end. Since the following code works

end = DbFunctions.AddSeconds((DateTime?)s.Start, 500)

and behaves as expected I must be doing something wrong with the conversion from TimeSpan to seconds.

What am I doing wrong in the first case?