As much as I feel like this is just covering up for other people's mistakes, I suppose it is possible that somebody adds a non-stackable item in the game, and for some reason, creates a recipe that contains multiples of that non-stackable item, which would then cause this error.
While the only other case where this would naturally occur is with armor and possibly vehicles/spidertrons (which I already have covered by other logic), I suppose I can add yet another ingredient check, just to ensure the game doesn't crash for people making silly decisions like this.
This change will likely increase loading times by some slight amount, but no clue how much; the lookups probably aren't too slow, and I didn't notice a difference even with Space Age enabled, so it's probably fine, but still thought I'd mention it.
Anyways, if a non-stackable item is detected, the returned item count will be clamped to 0 or 1, depending on efficiency and how many of the item were being used in the original recipe (it rounds up from 0.5 and clamps down to 1 when above that limit). Implemented in v9.0.37