Fails to build lighting. Lightmass crashed

Hi

I can’t build lighting on a level. I got this error log :


Error === Lightmass crashed: ===
Assertion failed: Importer.ImportData((FBaseMaterialData*)this) [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.7\Engine\Source\Programs\UnrealLightmass\Private\ImportExport\Material.cpp] [Line: 14]

KERNELBASE.dll!UnknownFunction (0x000007fefd9e940d) + 0 bytes [UnknownFile:0]
UnrealLightmass-Core.dll!UnknownFunction (0x000007fecb0699a4) + 0 bytes [UnknownFile:0]
UnrealLightmass-Core.dll!UnknownFunction (0x000007fecaf55888) + 0 bytes [UnknownFile:0]
UnrealLightmass-Core.dll!UnknownFunction (0x000007fecaf34e9b) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f63855c) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f5d91a0) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f63a650) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f698b6f) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f5e0775) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f639805) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f63abaf) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f60d1ee) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f641222) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f65c220) + 0 bytes [UnknownFile:0]
UnrealLightmass.exe!UnknownFunction (0x000000013f6adc37) + 0 bytes [UnknownFile:0]
kernel32.dll!UnknownFunction (0x00000000779c59ed) + 0 bytes [UnknownFile:0]
ntdll.dll!UnknownFunction (0x0000000077bfc541) + 0 bytes [UnknownFile:0]
ntdll.dll!UnknownFunction (0x0000000077bfc541) + 0 bytes [UnknownFile:0]


I am using a i7 with 8 Gb of RAM. Memory should not be an issue since it made it fine on a laptop with only 6 Gb of RAM.

Any ideas?