Apple Photos Migration failed, cannot be re-migrated

I migrated my Apple Photos library, but the migration failed, leaving me a couple of hundred images short (despite the count in Lightroom CC being greater than in Photos). I deleted the result of the failed migration—after all, the result is useless, especially when the failed photos aren't listed (or grouped in Apple Photos).

After doing a bit of cleanup in Apple Photos I decided to give it another try. But now it does not want to migrate again, as it is already migrated. That's not helpful. How can I get past this error?