LogWindows: Error: appError called: Assertion failed: IsThisNotNull(this, "UStruct::IsChildOf")

WeberSystems · August 19, 2025, 12:42pm

Fully blueprint project (RPG Engine). Encountered this error while working on the update. Engine version 5.6.1.

UATHelper: Cooking (Windows): LogWindows: Error: appError called: Assertion failed: IsThisNotNull(this, "UStruct::IsChildOf") [File:D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp] [Line: 2758]
UATHelper: Cooking (Windows): LogWindows: Error: begin: stack for UAT
UATHelper: Cooking (Windows): LogWindows: Error: === Critical error: ===
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error: Assertion failed: IsThisNotNull(this, "UStruct::IsChildOf") [File:D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp] [Line: 2758]
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc90c94028 UnrealEditor-Core.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8fe96472 UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc833e6eff UnrealEditor-Kismet.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc83415477 UnrealEditor-Kismet.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8fdd8e9d UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc90e7da72 UnrealEditor-Core.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc901d0a5c UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8fea99e4 UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc7e737803 UnrealEditor-BlueprintGraph.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8349d86a UnrealEditor-Kismet.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8349bcf2 UnrealEditor-Kismet.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8348677b UnrealEditor-Kismet.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc834f5528 UnrealEditor-Kismet.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc8b8d647b UnrealEditor-Engine.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc9028423c UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc90249bd9 UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc9024a25a UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc9024a7e1 UnrealEditor-CoreUObject.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc882391c5 UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc87f59898 UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc890622dc UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc89071744 UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc89070ecc UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc87cd9940 UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc87c9a69a UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffc87cb6c83 UnrealEditor-UnrealEd.dll!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ff747948fa0 UnrealEditor-Cmd.exe!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ff74793e39c UnrealEditor-Cmd.exe!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ff74793e6ba UnrealEditor-Cmd.exe!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ff74794209e UnrealEditor-Cmd.exe!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ff747954e44 UnrealEditor-Cmd.exe!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ff747957466 UnrealEditor-Cmd.exe!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error: [Callstack] 0x00007ffd0f52e8d7 KERNEL32.DLL!UnknownFunction []
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error: end: stack for UAT
PackagingResults: Error: appError called: Assertion failed: IsThisNotNull(this, "UStruct::IsChildOf") [File:D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp] [Line: 2758]
PackagingResults: Error: begin: stack for UAT
PackagingResults: Error: === Critical error: ===
PackagingResults: Error: Assertion failed: IsThisNotNull(this, "UStruct::IsChildOf") [File:D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp] [Line: 2758]
PackagingResults: Error: [Callstack] 0x00007ffc90c94028 UnrealEditor-Core.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8fe96472 UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc833e6eff UnrealEditor-Kismet.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc83415477 UnrealEditor-Kismet.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8fdd8e9d UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc90e7da72 UnrealEditor-Core.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc901d0a5c UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8fea99e4 UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc7e737803 UnrealEditor-BlueprintGraph.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8349d86a UnrealEditor-Kismet.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8349bcf2 UnrealEditor-Kismet.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8348677b UnrealEditor-Kismet.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc834f5528 UnrealEditor-Kismet.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc8b8d647b UnrealEditor-Engine.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc9028423c UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc90249bd9 UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc9024a25a UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc9024a7e1 UnrealEditor-CoreUObject.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc882391c5 UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc87f59898 UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc890622dc UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc89071744 UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc89070ecc UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc87cd9940 UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc87c9a69a UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffc87cb6c83 UnrealEditor-UnrealEd.dll!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ff747948fa0 UnrealEditor-Cmd.exe!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ff74793e39c UnrealEditor-Cmd.exe!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ff74793e6ba UnrealEditor-Cmd.exe!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ff74794209e UnrealEditor-Cmd.exe!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ff747954e44 UnrealEditor-Cmd.exe!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ff747957466 UnrealEditor-Cmd.exe!UnknownFunction []
PackagingResults: Error: [Callstack] 0x00007ffd0f52e8d7 KERNEL32.DLL!UnknownFunction []
PackagingResults: Error: end: stack for UAT
UATHelper: Cooking (Windows): Took 234,99s to run UnrealEditor-Cmd.exe, ExitCode=3
UATHelper: Cooking (Windows): Cook failed.
UATHelper: Cooking (Windows): (see C:\Users\User\AppData\Roaming\Unreal Engine\AutomationTool\Logs\D+Program+Files+Epic+Games+UE_5.6\Log.txt for full exception trace)
UATHelper: Cooking (Windows): AutomationTool executed for 0h 3m 56s
UATHelper: Cooking (Windows): AutomationTool exiting with ExitCode=25 (Error_UnknownCookFailure)
UATHelper: Cooking (Windows): BUILD FAILED

brs-sebascova · August 19, 2025, 2:36pm

Hello there @WeberSystems!

Checking through your log, it sounds like the cook procress is stumbling with a broken blueprint, considering the line below:

Assertion failed: IsThisNotNull(this, "UStruct::IsChildOf")
File: Class.cpp Line: 2758

UE is tacking with a null reference, most likely due a blueprint not properly updating to 5.6.1. So, the easiest fix would be to run “Validate Assets” on your Blueprint folders and watch for failures. After that, you can force a clean compile for any affected elements (select aBlueprints, right-click > Reload, then do Refresh All Nodes, recompile, save).

It would also help to clear your project’s cache, via closing UE, and deleting folders Intermediate/, DerivedDataCache/, and Saved/ from your main directory, then allowing UE to rebuild. After that, try to cook again.

WeberSystems · August 19, 2025, 10:40pm

Update: The error was caused by the GetClassDefaults node. Found it after 3 days of sorting through assets.