Both .icon and .icons are supported by the game, yet you check only for .icon in func.lua line 191. This causes a failure when you copy the icon & icon size over from the ingredient to the recipe, without setting .icons to nil, in case the size of the icons is less than the the new .icon_size.
Changing line 191 to
'if not recipe.icon and not recipe.icons then'
solves the issue.

Ahh, I see what you mean. Nice catch, that must have been a pretty difficult bug to find. Adding the fix to the next update

Happened when using with angelspetrochem as it adds recipes with .icons tag. Hard to trace to the root cause, but I managed.
For reference: https://github.com/Arch666Angel/mods/issues/20

Decided to upload it now, since I'm not actually sure when I can finish the next update. Many thanks for bringing this up to my attention and even fixing the issue.