Unexpected "cache lookup failed for collation 0" failure

Hi everyone,

Is the error "cache lookup failed for collation 0" in this context expected?

CREATE TABLE t0(c0 CHAR(2) COLLATE "C", c1 CHAR(2) COLLATE "POSIX");
INSERT INTO t0 VALUES('', '');
SELECT * FROM t0 WHERE t0.c1 NOT IN (t0.c0); -- unexpected: cache
lookup failed for collation 0

This looks like an internal error message to me. I would expect an
"ERROR:  could not determine which collation to use for string
comparison", which is shown in similar situations (e.g., when
replacing the "NOT IN" by "IN"). This appears like a minor issue to me
though.

Best,
Manuel