Null values where missing date

Hi, ChrisBroome , a most straightforward way is to enclose the function invocation into a try ... otherwise to catch errors like this,

Invoking = try Query1(#date(2020, 11, 10), null) otherwise ""

Hi ChrisBroome 

Try this:

(InitialDate as any, FinalDate as any ) as any =>
    if (InitialDate=null  or  FinalDate=null) then null else
        let
            DaysBetweenDates = Duration.Days(FinalDate-InitialDate),
            DaysList = List.Dates(List.Min({InitialDate,FinalDate}),Number.Abs(DaysBetweenDates)+1, Duration.From(1)),
            WeekDaysList = List.Select(DaysList, each (Date.DayOfWeek(_, Day.Monday) < 5) ),
            WorkingDays = (if DaysBetweenDates < 0 then -1 else 1) * List.Count(WeekDaysList)
        in
            WorkingDays

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Cheers