Ha! I knew it! Have to move the score update in `create_mod()`, because all properties of the new mod are `None` until it is committed, and Flask doesn't like adding or multiplicating with `None`.
A double commit works (although not that pretty):
```
    # Save database entry
    db.add(mod)
    db.commit()
    mod.score = get_mod_score(mod)
    db.commit()
```