Crash Error Code --> What does it mean?

<None> === Lightmass crashed: ===
Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:d:\build++ue4\sync\engine\source\runtime\core\public\Containers/Array.h] [Line: 611]
Array index out of bounds: 16740 from an array of size 16740

0x00007ff8d1639129 KERNELBASE.dll!UnknownFunction ]
0x00007ff838a8ed57 UnrealLightmass-Core.dll!UnknownFunction ]
0x00007ff838a916e7 UnrealLightmass-Core.dll!UnknownFunction ]
0x00007ff83892233a UnrealLightmass-Core.dll!UnknownFunction ]
0x00007ff8388ad689 UnrealLightmass-Core.dll!UnknownFunction ]
0x00007ff8388addd9 UnrealLightmass-Core.dll!UnknownFunction ]
0x00007ff7904eae48 UnrealLightmass.exe!DispatchCheckVerify<void,<lambda_9a6bd40285fd7859b4a5acda2249c4af> >() ]
0x00007ff79042ef0e UnrealLightmass.exe!Lightmass::FEmbreeGeometry::FEmbreeGeometry() ]
0x00007ff790437653 UnrealLightmass.exe!Lightmass::FEmbreeAggregateMesh::AddMesh() ]
0x00007ff7904841e4 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FStaticLightingSystem() ]
0x00007ff79043b1d4 UnrealLightmass.exe!Lightmass::BuildStaticLighting() ]
0x00007ff79045decd UnrealLightmass.exe!Lightmass::LightmassMain() ]
0x00007ff79047c87a UnrealLightmass.exe!main() ]
0x00007ff7904e6f38 UnrealLightmass.exe!__scrt_common_main_seh() ]
0x00007ff8d2a87974 KERNEL32.DLL!UnknownFunction ]
0x00007ff8d540a271 ntdll.dll!UnknownFunction ]

What do I have to change in order to fix that problem?