Passing multiple parameters to a function

I have a scalar function which looks up a value in a table:

FUNCTION [dbo].[fn_getLabor] 
(
	-- Add the parameters for the function here
	@ProjectUID as nvarchar(50)
)
RETURNS decimal(25,6) 
AS
BEGIN
	-- Declare the return variable here
	DECLARE @Labor as decimal(25,6)
	-- Add the T-SQL statements to compute the return value here
	  set @Labor = (SELECT SUM(NUM_VALUE)
                      FROM  MSP_TASK_CUSTOM_FIELD_VALUES
                      WHERE (PROJ_UID = @ProjectUID) AND MD_PROP_UID='FF8AB30F-6776-4737-A878-1739B2A1DF62')
	-- Return the result of the function
	RETURN @Labor 
END

I pass the ProjectUID to that function via a SSRS report using the call:

SELECT        dbo.fn_getLabor(@ScheduleParam) AS Expr1) AS TotalLabor

When I allow only one schedule to be chosen in the SSRS report via the @ScheduleParam, everything works perfectly.  When I allow multiple schedules to be chosen, I get an error from the function that too many arguments are specified.

Is there a way I can pass multiple parameters to the function so I can select multiple schedules in my SSRS report?  Do I need to re-write the function as an in-line function instead of using the scalar function?

Thank you for any suggestions,

DJ

DJ Johnson