SQL Server 2005 : Converting varchar value '1.23E-4' to decimal fails

declare @a varchar(40)
set @a='1.23e-4'
declare @b decimal(27,12)

if isnumeric(@a) =1
begin

    select @b=cast(@a as decimal(27,12))

end
else
begin
    select @b=-1
end

select @b

when exeucting above sql code under SQL 2005 environment I am getting following error.

Error converting data type varchar to numeric

anyone knows why?

thanks.