Versions (Be specific, do not write "latest"):

• Curios: 4.0.5.0
• Forge/Fabric: Forge 36.0.58
• Curious Elytra 4.0.2.3

Observed Behavior: Game crashes when I try to put an item into any curios slot.

Expected Behavior: Game should not crash.

Steps to Reproduce:

1. Open inventory.
2. Put Elytra (from Curious Elytra) into back slot.
3. Game crash
   ...

Crash Log: Please use a paste site like hastebin.
Minecraft just says "Unexpected game crash"

I can't be sure since I don't know your Caelus version, but this sounds like a similar issue I heard about so I think you just need to update Caelus to 2.1.3.0.