# LumenPT crash most likely issues with megalight

**URL:** <https://forums.unrealengine.com/t/lumenpt-crash-most-likely-issues-with-megalight/2854350>\
**Category:** Issues and Bug Reporting (Unreal Engine)\
**Tags:** unreal-engine, MegaLights, Lumen, Path-Tracing\
**Created:** [November 12, 2026, 7:29am UTC](https://forums.unrealengine.com/t/lumenpt-crash-most-likely-issues-with-megalight/2854350 "2026-11-12T07:29:26Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![John70176162](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@John70176162](https://forums.unrealengine.com/u/John70176162)\
**Post date:** [November 12, 2026, 7:29am UTC](https://forums.unrealengine.com/t/lumenpt-crash-most-likely-issues-with-megalight/2854350/1 "2026-11-12T07:29:26Z")

</div>

### Summary

When i enabled LumenPT in unreal engine 5.9 and restarted my project the project could not compile shaders and crashed

### What Type of Bug are you experiencing?

Editor

### Steps to Reproduce

LumenPT supported cvar in the defaultengine.ini with megalights and world cache enabled. restart the editor

### Expected Result

the project will crash at splash window and throw some compilation errors and ask you to either retry or give up and close the project

### Observed Result

same

### Affects Versions

5.9

### Platform(s)

Windows

### For crash reports, include your callstack

LoginId:db2db5494207b97e9fda2caade15ced5  
EpicAccountId:5bf9c3cd12244acd8c056b1402eca587

Fatal error: [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\ShaderCompiler\ShaderCompilerEditor.cpp] [Line: 599]  
6 errors encountered compiling global shaders for platform PCD3D\_SM6:  
Shader debug info dumped to: “D:\Unreal Engine Projects\KB3D\_NeoCities\_UE4Project\Saved\ShaderDebugInfo\PCD3D\_SM6\Global\FLumenPTVisualizeHardwareRayTracingRGS\0”

C:\UE\_5.9\UE\_5.9\Engine\Shaders\Private\MegaLights\MegaLightsShading.ush(66,47): Shader FLumenPTVisualizeHardwareRayTracingRGS, Permutation 0, VF None:  
/Engine/Private/MegaLights/MegaLightsShading.ush(66,47): error: no matching function for call to ‘UnpackSubstrateHeaderIn’  
FSubstratePixelHeader SubstratePixelHeader = UnpackSubstrateHeaderIn(Substrate\_MaterialTextureArray, SubstrateAddressing, Substrate\_TopLayerTexture);  
^ ~~~~~~~~~~~~~~~~~~~~~~  
C:\UE\_5.9\UE\_5.9\Engine\Shaders\Private\MegaLights\MegaLightsShading.ush(119,51): Shader FLumenPTVisualizeHardwareRayTracingRGS, Permutation 0, VF None:  
/Engine/Private/MegaLights/MegaLightsShading.ush(119,51): error: no matching function for call to ‘SubstrateDeferredLighting’  
FSubstrateDeferredLighting SubstrateLighting = SubstrateDeferredLighting(  
^ ~~~~~~~~~~~~~~~~~~~~~~~~  
Shader debug info dumped to: “D:\Unreal Engine Projects\KB3D\_NeoCities\_UE4Project\Saved\ShaderDebugInfo\PCD3D\_SM6\Global\FLumenPTInitialSamplingRGS\0”

C:\UE\_5.9\UE\_5.9\Engine\Shaders\Private\LumenPT\LumenPTInitialSampling.usf(122,86): Shader FLumenPTInitialSamplingRGS, Permutation 0, VF None:  
/Engine/Private/LumenPT/LumenPTInitialSampling.usf(122,86): error: no member named ‘GBuffer’ in ‘FMegaLightsMaterial’  
FMaterialSample MaterialSample = SampleMaterial(RandSequence, PathState.Material.GBuffer, PathState.ViewDirection, DebugContext);

```auto
C:\UE_

UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_Engine
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor_CoreUObject
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
kernel32
ntdll
```