BEGIN TRANSACTION error when a R/O sqlite database is attached to a R/W database

I have two sqlite databases. I open my main DB which is in R/W memory (iOS app Documents folder) and then ATTACH a second DB that is in R/O memory (iOS app Bundle).

Whenever I do BEGIN TRANSACTION, I immediately get "attempt to write a readonly database" even though I have no intention of changing the DB that's in readonly memory. Is that error expected? Is there anything I can do short of temporarily DETACH-ing to get the BEGIN TRANSACTION to work? (I'm using FMDB.)