Received a crash when trying to use a belt balancer blueprint.

Error while running event companion-drones-mjlfix::on_pre_build (ID 9)
LuaItemStack doesn't contain key contents.
stack traceback:
[C]: in function 'index'
__companion-drones-mjlfix/script/companion.lua:2252: in function <companion-drones-mjlfix/script/companion.lua:2240>
(...tail calls...)
companion-drones-mjlfix/script/companion.lua:2281: in function 'get_blueprint_area'
companion-drones-mjlfix/script/companion.lua:2314: in function 'handler'
core/lualib/event_handler.lua:72: in function <core/lualib/event_handler.lua:70>

+1

Huge lag spike on placing blueprints

+1

I found a work around, first disable auto build of the companion then place the blueprint and turn in back on

Gotcha. I didn’t even have a companion out at the time but that will help others

I'm gonna be totally honest with y'all: almost 100% of the blueprint code comes from the original mod author, and it breaks almost every time I poke at it. So, unfortunately, any blueprint related issues are most likely here to stay.

Hi MrJakobLaich / Maoman,

I saw the earlier comments here:

• ashragnorok reported the crash with a belt balancer blueprint
• Fortheking55 mentioned huge lag spikes when placing blueprints
• Raukie found the workaround: disable companion auto-build, place the blueprint, then turn it back on
• Maoman wrote that the blueprint code mostly comes from the original author and tends to break whenever touched

We hit the same crash on Factorio 2.0.77 with companion-drones-mjlfix 3.1.2:

Error while running event companion-drones-mjlfix::on_pre_build (ID 9)
LuaItemStack doesn't contain key contents.

Stacktrace:

companion-drones-mjlfix/script/companion.lua:2252
in get_all_blueprint_entities()
called from get_blueprint_area()
called from on_pre_build

The workaround from Raukie also makes sense with what we saw: the crash only happens when companion construction / auto-build is active and the mod tries to inspect the blueprint in on_pre_build.

We tested a minimal defensive fix on our server. The goal was not to rewrite the fragile blueprint logic, only to avoid the server crash.

Root cause seems to be this line:

for _, item in pairs(blueprint_record.contents) do

That works when the object exposes contents, but crashes when the cursor blueprint/book is represented as a LuaItemStack or item-backed blueprint book.

Our tested fix was:

1. Add a small helper like get_blueprint_book_children(record).
2. First try record.contents through pcall.
3. If that fails, try record.get_inventory(defines.inventory.item_main) and iterate valid inventory slots.
4. Replace direct blueprint_record.contents iteration with that helper.
5. Wrap calls to get_all_blueprint_entities() from get_blueprint_area() in pcall, so unknown cursor/blueprint states get ignored instead of crashing the whole server.

After this patch:
- the server no longer crashes on the problematic blueprint
- companion construction still works for our tested case
- if the mod cannot understand a blueprint object, it falls back safely instead of killing the multiplayer session

So this may be a low-risk guard around the existing blueprint code rather than a major blueprint rewrite.

I can provide the exact diff if that helps.