Q On Joining Tables With Nullable Fields

Question.
I have a new table that I am adding to a script that I wrote. This
table has 3 fields, the first 2 fields are used in the on statement as
being = other fields in the script.

The first field always has data in it, but the 2nd field is sometimes
null.

So my problem is if both fields have data in them and they both match
to the data in the fields that I am linking them to, then it returns
the 3rd field without a problem. However if the 2nd field is null then
it is returning a null for the 3rd field. I have checked and the field
that I am linking to is null also.

So if I have

select t1.field1, t1.field2, t2.field1, t2.field2, t2.field3
from table1 t1
join table2 t2
on t1.field1=t2.field1 and t1.field2=t2.field2

with 2 records in each table
table1: record1: data, data
record2: data, null
table2: record1: data,data,data
record2: data,null,data

what I get from the script is
record1: data, data,data,data,data
record2: data,null,data,null,null


I would expect
record2: data,null,data,null,data

I hope this makes sense, I didn't want to post the entire actual script
as it is about 150 lines long.

Thanks in advance.