Nz Function In Business Days HELP!

I have a query that calculates business days. However if my end date is missing the query has error codes because the date its looking for is null. Can show me were to put the nz function.

here's the module for the business days

Function Work_Days(BegDate As Variant, EndDate As Variant) As Integer

' "Calculating the workdays between Dates"
' Note that this function does account for holidays.

Dim WholeWeeks As Variant
Dim DateCnt As Variant
Dim EndDays As Integer

BegDate = DateValue(BegDate)
EndDate = DateValue(EndDate)
DateCnt = BegDate
EndDays = 0

Do While DateCnt <= EndDate

If Not IsNull(DLookup("HoliDate", "tblHolidays", "[HoliDate]=#" & DateCnt & "#")) Then
EndDays = EndDays - 1
End If

If Format(DateCnt, "ddd") <> "Sun" And _
Format(DateCnt, "ddd") <> "Sat" Then
EndDays = EndDays + 1
End If
DateCnt = DateAdd("d", 1, DateCnt)
Loop
Work_Days = EndDays


End Function

and here's the query

SELECT [Reques_ID], Work_days([Date_Received],[To_Director]-1) AS Turnaround_To_Director,
Work_days([To_Director],[From_Director]-1) AS Turnaround_From_Director, Work_days([To_VP],[From_VP]-1) AS Turnaround_From_VP, Work_days([Position_Number_Requested],[Position_Number_Recieved]-1) AS Turnaround_For_Position_Number, Work_days([Date_Received],[Approval_to_mgr]-1) AS Turnaround_EPS, Work_days([Date_Received],[JOIS_Posted_Date]-1) AS Turnaround_To_Posting, Work_days([Approval_to_mgr],[JOIS_Package_Return_Date]-1) AS Turnaround_For_Package_Return
INTO tbl_turnaround
FROM Artifact;