Conversion failed when converting datetime from character string.

When I exec to search, I am getting an error message:
"Conversion failed when converting datetime from character string."
Which I couldn't understand. I checked everything and make sure that datatype is correct.
Please help me.


Create Proc SearchBoard (

      @RegisterDateFrom      datetime      = Null
,      @RegisterDateTo            datetime      = Null
)
AS
Set NoCount ON

If @RegisterDateFrom is not null
      Select BoardID      From Board
      where RegisterDate >='@RegisterDateFrom' and RegisterDate <'@RegisterDateTo'
      Order by RegisterDate Desc
--------------------------------------
exec searchboard @RegisterDateFrom='12-01-2007', @RegisterDateTo='12-02-2007'Start Free Trial