Error Selecting Date Convert Fields With Nulls

I have a select statement which has to convert some date fields stored
as varchar based on critera of adding numbers to the date. What I have below
will return data show below but gives me an error message when it encounters
any data with null in the field. Is there something wrong in my conversion?
Thanks


declare @tdate varchar(10)
declare @rldate datetime
select @tdate = 20000630
select @rldate = @rldate + 2
select patient_.df_admit_date, patient_.df_ppdb_date from patient_
where (convert(datetime, df_ppdb_date, 101)+ 12) >= @tdate or
(convert(datetime, df_admit_date, 101) + 2 ) >= @tdate

19980910 20000626
19981215 20000627
19590114 20000621
20000629 NULL
20000706 NULL
20000711 NULL

(6 row(s) affected)

Server: Msg 242, Level 16, State 3, Line 5
The conversion of a char data type to a datetime data type resulted in an out-of-range datetime value.