Sequences, triggers and 'OLD' - am I being stupid?

I'll admit straight away I'm a novice, but this one has me perplexed....

I have a table, T1, with a column (userid) that is an int4 generated from a
sequence.

I have a second table (T2) that I wish to use to hold audited values from
T1.

I have created a trigger function written in PL/PGSQL, set to be for each
row after update on T2, which is supposed to copy the OLD values into T2.

The snag is that when I try to update a row in T1 I get:

ERROR:  record "old" has no field "userid"

This behaviour does not seem to be documented anywhere. Am I just being
dense?

TIA,

Steve S

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