Conversion failed when converting the nvarchar value 'NULL' to data type int with Replace function only

Question

I have three tables in my database with the following properties:

create table #Step_1    
    (
        ID int identity,
        Recipient_ID varchar(50),
        Diag_ID varchar(50),
        Diag_Group varchar(50),
        Class Int,
        [Rank] Int
    )
  
create table #tmp_indx    
    (
        Recipient_ID    varchar(50),
        DIAG_ID         varchar(50),
        DIAG_ID_UPDATED varchar(50)
    )

update #tmp_indx    
Set DIAG_ID_Updated = REPLACE(DIAG_ID, '.','')

create table #cdps_format
    (
        DIAG_ID     varchar(50),
        DIAG_Group      varchar(50),
        Class           varchar(50),
        Rank            varchar(50)
    )
  

So now when I run this query, everything runs smoothly

insert into #Step_1 (  Recipient_ID,
            Diag_ID,
            Diag_Group,
            Class,
            [Rank]
                )
select  Recipient_ID,
    Diag_ID,
    Diag_Group,
    Class,
    [Rank]
from 
(select a.Recipient_ID,
    a.DIAG_ID,
    b.Diag_Group,
    b.Class,
    b.[Rank],
    ROW_NUMBER() OVER (PARTITION BY Recipient_ID, Class ORDER BY [Rank]) AS rn
from #tmp_indx a
inner join #cdps_format b
on a.DIAG_ID = b.diag_id) d
where d.rn = 1
order by recipient_id, diag_id

But as soon as I change a.DIAG_ID to a.DIAG_ID_UPDATED in the inner join, I get the error:

Conversion failed when converting the nvarchar value 'NULL' to data type int

Does anyone have any idea why this happens? From what I understand, DIAG_ID_UPDATED is just DIAG_ID but removed any '.' from the variable, so I am a bit confused why this is happening. I thought it would be that both rank and class are characters in #cdps_format and that they are INT in #step_1, but that doesn't really explain to me why the error only occurs after changing the variable in the join. Thanks!