Assertion Failed

[rECOMBINAToR]

Hi Guys,

I got this error continuously, ‘kinda’ randomly:

[2025.12.30-19.09.36:566][162]LogWindows: Error: appError called: Assertion failed: SyncPosition.NextMarkerName != NAME_None [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\Animation\AnimSequence.cpp] [Line: 4307]

I cannot find any clue what to check. I got this error mostly in editor, but my friend managed to recreate in a packaged project. It mostly occurs when I’m rolling after landing, but not every time. My friend did not roll at all and managed to recreate.

I know that complete solution cannot be given without the whole project, but I need only a direction where to start.

How can I find this AnimSequence.cpp file to check the line that the log mentions? Should I check the Animation Blueprint, the animation sequences or what? How should I get a more precise error message?

Thank you in advance.

[Mind-Brain]

@rECOMBINAToR

LogWindows: Error: appError called: Assertion failed

Hey @rECOMBINAToR!

Try this thread! It’s sounding like an NRE or corruption somewhere, as the AnimSequence.cpp is an engine file.

Hope that helps!

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

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 affect…

[rECOMBINAToR]

@Mind-Brain

AnimSequence.cpp

Hi @Mind-Brain !

Thank you for your answer. The problem was not the same as mine but it was a really good starting point.

I’ve dug deeper and found the AnimSequence.cpp. It was in the engine folder (same place where the error log said). However, it was not clear what is causing the “SyncPosition.NextMarkerName != NAME_None“ error, so I went to check my animations.

I’ve found some of my Blend Spaces were in a custom Sync Group, but my custom Animation Montages were not in this group.

First, I’ve delete the custom group from that blend space, but it caused other issues, due to the lacking of sync. I was unable to recreate the original error, but the not synchronized animatons were wrong, so I put back the group.

I’ve added the sync group to the montage where I’ve discovered the error.

I think using different sync groups may cause this kind of errors. It hardly depends on the framerate, becasue I was unable to recreate the error with low FPS. Moreover, my friend tested the packaged game, and he got the error doing different stuff. While I’ve managed to cause the error by rolling shortly after landing (only wtih more than 60 FPS) he just did some jumps and attacks and randomly, the error occured.

This discovery led me to check the landing animation, which handled by the mentioned Blend Space.

Now I’m checking all of my animations that connect to an animation which is in a group. Removing the groups may solve the problem, but all animation must be checked to see what happens without the sync.

Also, it worth to check the unreal documentation about sync groups:

https://dev.epicgames.com/documentation/en-us/unreal-engine/animation-sync-groups-in-unreal-engine?application_version=5.1