nod: Will-o'-the-Wisps updated.
when dealing damage to a Firefly, write message:
error while running event Turret-Shields::on_entity_damaged(ID 97)
Turret-Shields/control.lua:512:attempt to index fields"?"(a nil valume

It's a will-o-wisp, not firefly! :)

Anyway, such damage can be special in two ways:

• Has damage-type=corrosion, which was added in data.lua of that mod.
• Damage source can be essentially a cloud object ("smoke-with-trigger"), doing damage when passing over turret, not a unit like biter.

I'd suspect the first one, as it's probably rare for a mod to add its own damage type, but didn't look at the code or test myself to confirm.

@DENGERREDDS:
You can try disabling "Wisps can attack" and "Purple wisps corrode buildings" in Mod Options for will-o-wisps mod as a temporary workaround, as that should make them never cause any damage to turrets or any other objects, eliminating this whole scenario.

Forgot to mention:
In data-final-fixes.lua, mod goes through all prototypes and sets them to be immune to corrosion, except for some categories and special-case prototypes.
"electric-turret" category is patched to have corrosion resistance equal to acid resistance in particular, so this new damage type might also be found there, in case unexpected key in resistances can be the issue.

fixed i guess.