Hi Joel,
I had this same error message once, and asked about it on the list. There's some discussion if you search the archives. I can't recall that there was any good resolution of the topic.
It is suspicious that the erroring line has a grace at the start. Can you try applying a grace spacer rest of the same duration in the bass clef at the same musical moment? I have a feeling this may be the famous grace note bug that lilypond has.