ODP.NET :: Entity Framework Concurrency - UpdateException

When running into what should be an OptimisticConcurrencyException in entity framework, i'm getting an update exception as follows:

"A null store-generated value was returned for a non-nullable member 'DateChanged' of type 'FDDT.Model.FeedArray'."

I'm using the DateChanged field as the concurrency tracking field. In entity framework DateChanged has the following properties:
StoreGeneratedPattern: Computed
Concurrency Mode: Fixed

I have a trigger set on this field as follows:

TRIGGER FEED_ARRAY_TRIG_DTCHG before update on FEED_ARRAY for each row
begin
:new.DATE_CHANGED := sysdate;
end;

I've tried setting this concurrency tracking field to "Identity" instead, however that causes another error when I run into a concurrency exception and refresh using the client wins option.