Oh that could happen in my data. Somewhat unlikely (that none of at least 100 words in a reddit post were in vocabulary), but  definitely possible. I’ll look for that.
Is there a good way to find out what datum is causing the issue? (Other than knowing the problem and searching input) I haven’t found many examples using dataloaders… – I think I’ve answered my own question and realized that I really should be writing in a better debugging environment, but if you have other suggestions I’d love to hear them.