FIll in missing months

I am querying a table that includes the rider id, the date of the ride and the distance. Problem is that they may not have ridden I In a given month, so I want to include that month with a distance of 0, so all twelve months are listed. Here s my attempt... however it doesn't pick up the zero moths.

alter PROCEDURE dbo.UM_getmonthlymiles  
        -- Add the parameters for the stored procedure here
        @YEAR INT
        , @RIDER INT
        
AS
BEGIN

declare @months table
(
    monthnum int
 --,  mnthname varchar(3)
)
insert into @months  

select MonthInt
from (
select 1 as MonthInt union all select 2  union all  select 3  union all   select 4 union all 
select 5 as MonthInt union all select 6  union all  select 7  union all   select 8 union all 
select 9 as MonthInt union all select 10  union all  select 11  union all   select 12 
)M

select* from @months

declare @mnthmiles table
(
  distance float
,  riderid int
--, mnthname varchar(10)
, monthnumber int
)
        -- SET NOCOUNT ON added to prevent extra result sets from
        -- interfering with SELECT statements.
        SET NOCOUNT ON;
insert into @mnthmiles
    select isnull(sum(distancemiles),0), @RIDER,  isnull(DATEPART(mm,ridedate), M.monthnum)--) --as [num}
from mileagelog 
LEFT JOIN @months M
ON M.monthnum = DATEPART(mm,ridedate)
where DATEPART(yyyy,ridedate)  = @year 
and riderid = @rider
group by DATEPART(mm,ridedate), riderid, DATENAME(mm,ridedate), DATEPART(mm,ridedate), M.monthnum order by DATEPART(mm,ridedate),  M.monthnum 
select * from @mnthmiles