F#, Entity Framework and nullable values

Question

This is an extremely difficult problem in F#, that is extremely easy in C#

I'm using a custom DomainManager to write an Azure Mobile App Table Controller in F#. The DomainManager inherits from MappedEntityDomainManager<,>, which uses Automapper under the covers to map my database objects into mobile-friendly records.

The Automapper profile looks like this:

type EventOrganiserProfile() =
    inherit Profile()
    do
        let nullDateTime = Nullable<DateTime>()
        base.CreateMap<MyDbEntity, MyMobileDto>() 
            ...
            .ForMember(fun dest -> dest.UpdatedAt, fun opt -> opt.MapFrom(fun src -> if src.CancelledUtc <> nullDateTime then src.CancelledUtc.Value else src.StartedUtc))
            ...
        |> ignore         
        base.CreateMap<MyMobileDto, MyDbEntity>() 
            ...
        |> ignore       

src.StartedUtc is a DateTime, src.CancelledUtc is a nullable DateTime, and because its class inherits from EntityData, dest.UpdatedAt is a nullable DateTimeOffset.

This code compiles, but when I try to run it, I get the error message

System.NotSupportedException: 'LINQ to Entities does not recognize the method 'System.DateTime Invoke(System.Nullable`1[System.DateTime])' method, and this method cannot be translated into a store expression.'

What this seems to be referring to is my use of the .Value property of Nullable<DateTime>. The underlying cause is F#'s rigorous type-checking. In C# this would have been handled seamlessly by the null coalescing operator ??.

Is there a way to simulate null coalescing as part of an Entity Framework-friendly expression in F#?