[sql] Alter a SQL server function to accept new optional parameter

I already have a function in SQL Server 2005 as:

ALTER function [dbo].[fCalculateEstimateDate] (@vWorkOrderID numeric)
Returns varchar(100)  AS
Begin
  <Function Body>
End

I want to modify this function to accept addition optional parameter @ToDate. I am going to add logic in function if @Todate Provided then do something else continue with existing code.

I modified the function as:

ALTER function [dbo].[fCalculateEstimateDate] (@vWorkOrderID numeric,@ToDate DateTime=null)
Returns varchar(100)  AS
Begin
  <Function Body>
End

Now I can call function as:

SELECT dbo.fCalculateEstimateDate(647,GETDATE())

But it gives error on following call:

SELECT dbo.fCalculateEstimateDate(647)

as

An insufficient number of arguments were supplied for the procedure or function dbo.fCalculateEstimateDate.

which as per my understanding should not happen.

Am I missing anything? Thanks in advance.

I have found the EXECUTE command as suggested here T-SQL - function with default parameters to work well. With this approach there is no 'DEFAULT' needed when calling the function, you just omit the parameter as you would with a stored procedure.

The way to keep SELECT dbo.fCalculateEstimateDate(647) call working is:

ALTER function [dbo].[fCalculateEstimateDate] (@vWorkOrderID numeric)
Returns varchar(100)  AS
   Declare @Result varchar(100)
   SELECT @Result = [dbo].[fCalculateEstimateDate_v2] (@vWorkOrderID,DEFAULT)
   Return @Result
Begin
End

CREATE function [dbo].[fCalculateEstimateDate_v2] (@vWorkOrderID numeric,@ToDate DateTime=null)
Returns varchar(100)  AS
Begin
  <Function Body>
End