Impossible to insert serialized data with update_sql()

I just lost way more time than I would've liked on this.

I have an array, which when serialized looks like this:

a:1:{s:6:"groups";a:1:{i:2237;s:4:"2237";}}

However, when passed into update_sql(), it gets turned into this:

a:1:s:6:"groups";a:1:i:2237;s:4:"2237";

This is of course meaningless and causes the data when unserialized again to be invalid. A workaround is to use db_query() instead, but then you can't see the query in the list of those that have run.

Thankfully, this god-awful function is finally gone in D7, so this is only an issue in D5 and D6. I figure this might be a duplicate of an existing bug, since there seem to be blog posts documenting the problem, but I didn't find any in searching.