SQL update a column from another table, matching RRNs

[Dan <dan27649@xxxxxxxxx>]

I messed up a column of my test data.  I have another table that has the
correct data I would like to fix the original table with.  Since the column
was a key field I messed up, I have to match the records by RRN.  (Other
values in the matching records are different, and this is why I can't just
do a straight CPYF.)  I've tried several variations of the following:

update Lib1/Customer a
   Set a.cmcust = (select z.cmcust from Lib2/Customer z
                              where RRN(a) = RRN(z)       )

In all cases, I'm getting SQL5001 (Column qualifier or table A undefined.)

I know I can write an RPG program to do this in probably 5 minutes, but am
asking here to try to expand my working knowledge of SQL.

TIA,
- Dan