How to Fix “Assertion failed: (Index >= 0) & (Index < ArrayNum)” in Unreal Engine

This assertion is UE's bounds check catching an invalid TArray access — a guaranteed crash in shipping if unchecked. The stack trace names the function; the index came from somewhere staler. Here is the hardening.

How to fix it

1. Guard with IsValidIndex

Before Items[i], check Items.IsValidIndex(i) anywhere the index is not provably fresh — especially handlers that run frames after the index was computed.

2. Remove while iterating correctly

Use reverse loops or RemoveAllSwap with predicates; forward-iterating removal shifts elements and invalidates every held index.

3. Distrust cross-boundary indices

Indices arriving via RPC, save games, or UI callbacks meet different array states — clamp or look up by ID instead of raw position.

4. Keep the assert meaningful

Do not silence with unchecked GetData()[i] access; the assert is what stands between you and silent memory corruption.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unreal Engine error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.