Re: What do I do with this error?

BTW, are you certain that your failure case actually does involve a
NULL, and not an empty string?

regression=# select 'foo' ~* null;?column?
----------

(1 row)

regression=# select 'foo' ~* '';
ERROR:  regcomp failed with error empty (sub)expression

This is correct according to the POSIX definition of regular expressions
("A (modern) RE is one or more non-empty branches ...").
        regards, tom lane