What causes "index must be between 0 and {lightmapCount - 1}, but was {index}" in Unity-Technologies/UnityCsReference?

Bound the loop with lightmapRequests.lightmapCount.

Unity-Technologies/UnityCsReference · error · ArgumentException

index must be between 0 and {lightmapCount - 1}, but was {in

Error message

index must be between 0 and {lightmapCount - 1}, but was {index}

What it means

Thrown by LightmapRequests.lightmapInstanceCount when the uint lightmap index is >= lightmapCount. The lightmap index space is separate from instance, texture, and request indices; only lightmapCount is a valid bound. The method returns how many instances are baked into the given lightmap.

Source

Thrown at Editor/Mono/GI/LightBaker.bindings.cs:858

            if (_ownsPtr && _ptr != IntPtr.Zero)
            {
                Internal_Destroy(_ptr);
                _ptr = IntPtr.Zero;
            }
        }

        public extern ulong GetByteSize();
        static extern IntPtr Internal_Create();
        [NativeMethod(IsThreadSafe = true)]
        static extern void Internal_Destroy(IntPtr ptr);

        public extern LightmapRequest[] GetLightmapRequests();
        public extern void SetLightmapRequests(LightmapRequest[] requests);

        public uint lightmapInstanceCount(uint index)
        {
            if (index >= lightmapCount)
                throw new ArgumentException($"index must be between 0 and {lightmapCount - 1}, but was {index}");
            return Internal_InstanceCount(index);
        }

        public extern uint lightmapCount { get; }
        extern uint Internal_LightmapWidth(uint index);
        extern uint Internal_LightmapHeight(uint index);
        extern uint Internal_InstanceCount(uint lightmapIndex);
        public extern void SetLightmapResolution(Resolution resolution);
        public extern void SetSingleLightmapResolution(uint index, Resolution resolution);
        public Resolution lightmapResolution(uint index)
        {
            if (index >= lightmapCount)
                throw new ArgumentException($"index must be between 0 and {lightmapCount - 1}, but was {index}");
            Resolution resolution;
            resolution.width = Internal_LightmapWidth(index);
            resolution.height = Internal_LightmapHeight(index);
            return resolution;
        }

View on GitHub (pinned to 225b0fbdb5)

Solutions

Bound the loop with lightmapRequests.lightmapCount.
Re-query lightmapCount after any SetLightmapRequests call before iterating.
Guard: if (index < lmRequests.lightmapCount) { ... }

Example fix

// before
for (uint i = 0; i < requests.Length; i++)
    n += lmRequests.lightmapInstanceCount(i);
// after
for (uint i = 0; i < lmRequests.lightmapCount; i++)
    n += lmRequests.lightmapInstanceCount(i);

Defensive patterns

Strategy: validation

Validate before calling

if (index < lmRequests.lightmapCount)
{
    uint n = lmRequests.lightmapInstanceCount(index);
}

Prevention

Bound lightmap iteration with lightmapCount.
Re-query lightmapCount after SetLightmapRequests before iterating.
Do not pass renderer.lightmapIndex directly as the argument.

When it happens

Trigger: Iterating lightmaps with the instance count, the LightmapRequest[] length, or a scene's lightmap index from renderer.lightmapIndex; calling after SetLightmapRequests changed lightmapCount.

Common situations: Readback code that assumes lightmapCount equals the number of renderers or scenes; stale LightmapRequests reused after the request set changed.

Related errors

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/48870b0226a6dd23. Report an issue: GitHub.