I have a table T (PBM_CLAIM BIGINT, CLAIM_SRC SMALLINT, ...) with the
unique constraint:

ALTER TABLE RELAY_CLAIM ADD CONSTRAINT U_RELAY_CLAIM_SRC UNIQUE
(PBM_CLAIM,CLAIM_SRC);

If I try to insert a row with (NULL, NULL, ...) when this constraint
exists, I get the error

ISC ERROR CODE:335544334
ISC ERROR MESSAGE:
conversion error from string " "

I drop the constraint, and the insert works.
I'm at a loss to understand what's going on, as adding the constraint to
an existing table works and there are thousands of rows where the tuple
is (NULL,NULL).
A simple test table with only a few test rows, these two columns, and
the unique constraint works fine.

Does anyone have any ideas or experience anything like this?