Метод F# возвращает null вместо Option

Разрабатываю F# приложение .net 4.6.1 на VS2015. у меня есть методы:

type CommonHelper = 
    static member SideEffectOnNull act x = if x = null then act(); x else x
    static member SideEffectOnNotNull act x = if x <> null then act(); x else x

...

    static member GetPerformanceCounter ( fname: CounterFullName ) = 

        let getCounterInternal ( counterFullName: CounterFullName ) =
            Log.Information("Getting counter: {category}\\{name}\\{instance} ",  counterFullName.Category, counterFullName.Name, counterFullName.Instance)
            let receivedCategory = PerformanceCounterCategory.GetCategories().FirstOrDefault( fun x -> String.Equals( x.CategoryName, counterFullName.Category.Category, StringComparison.InvariantCultureIgnoreCase ) )
            if receivedCategory = null  then
                Serilog.Log.Warning ( "Category not found: {category}", counterFullName.Category ); null
            else
                let receivedCounters = PerforrmanceCounterProxy.GetPerformanceCountersFromCategoryOrNull counterFullName.Instance receivedCategory
                if receivedCounters = null then 
                    Log.Warning ("Instance not found {name}(instance: {instance}) in category {category}", counterFullName.Name, counterFullName.Instance, counterFullName.Category); null
                else
                    receivedCounters.FirstOrDefault( fun y -> String.Equals( y.CounterName, counterFullName.Name.Name, StringComparison.InvariantCultureIgnoreCase ) ) 
                    |> CommonHelper.SideEffectOnNull ( fun unit -> Log.Warning ("Name {name}(instance: {instance}) not found for category {category}", counterFullName.Name, counterFullName.Instance, counterFullName.Category) )

        getCounterInternal fname
        |> CommonHelper.SideEffectOnNull (fun unit ->Log.Warning( "Getting counter failed: {category}\\{name}\\{instance}", fname.Category, fname.Name, fname.Instance )) 
        |> CommonHelper.SideEffectOnNotNull (fun unit ->Log.Information( "Getting Counter secceed: {category}\\{name}\\{instance}", fname.Category, fname.Name, fname.Instance ))
        |> (fun x -> if x = null then None else Option.Some(x)) 

Но когда я вызываю этот метод, я получаю null вместо option. Что я делаю не так?