[newbie] function question

SQL Server 2005. The function below compiles fine. If the two dates
are same return 1 (true) else false.

But the testing gives an type error
> execute issameday getdate, getdate
error -> Error converting data type nvarchar to datetime.
Works for
> execute issameday '08/08/08', '08/08/08'

I don't understand what is going on :(  help please


create function IsSameDay (
    @[EMAIL PROTECTED]
 datetime,
    @[EMAIL PROTECTED]
 datetime = getdate
)
returns bit
as
begin

    declare @[EMAIL PROTECTED]
 int
    set @[EMAIL PROTECTED]
 = datediff(day, @[EMAIL PROTECTED]
 @[EMAIL PROTECTED]
)

    if @[EMAIL PROTECTED]
 = 0 return 1
    return 0

end
go