Probably simple sql question

I need to do a large update and can't quite figure it out. Essentially,

I have two tables of the same name and fieldtypes in two different schemas.
The only difference is that on one table, a field called "offset" has values in
it and on the other table (table B), it doesn't. The idea is to get table b to
look exactly like table a.

I tried

Update B set offset =
(select offset from A where primarykey = B.primarykey)

and got
cannot update mandatory (NOT NULL) column to NULL. The offset is not null on both tables, so there are no null values. My correlation
logic must be askew.

Would appreciate any advice,

• Dc.