BuiltData Serial Size Mismatch

I am having trouble with my lightingbuild data for only one of my levels.

I can build lighting fine, however when I close the editor, or change to a different map and back, the editor crashes. I checked the logs and when the engine first crashes this line appears

Fatal error: [File:D:\Build++UE4+Release-4.15+Compile\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\LinkerLoad.cpp] [Line: 3498]
LogWindows:Error: MapBuildDataRegistry /Game/Maps/FulhamReachApartment_BuiltData.FulhamReachApartment_BuiltData: Serial size mismatch: Got -1694559444, Expected -1694559444

I can delete the BuildData and open the map and rebuild the lighting again however the same thing happens. If i replace the lighting build with an autosave or a copy of the Build Data before the crash I get this error:

Attempted to seek to a negative location (-1694542075/2652971352), file: …/…/…/…/…/…/Users/Paul/Desktop/Fulham Reach VR/UE/FulhamReachVR/Content/Maps/FulhamReachApartment_BuiltData.uasset. The file is most likely corrupt.

This does not happen on any other levels in the project and I have run out of options which I can think of.

Any help or route suggestions would be appreciated, I will be posting this question on UDN also.


Hi Michael,

To clarify, you can save successfully, just not open the map once you’ve saved it? This sounds like a serialization error. If the order or size of data saved does not match the order or size of data loaded, you get that type of error.

It seems very unlikely to me that such a blatant error exists in the engine out of the box - we would have caught it otherwise as we are saving and loading maps with built data regularly. Have you made any source modifications that might be relevant? If so, does the crash still repro if you revert them? Can you reproduce the crash in a new project with a simple level and upload it for me?

Another way to get a serialization mismatch is to load newer content with an older executable. The newer content will have been saved with data in a certain order, that the older executable doesn’t know about.

Thanks for the reply Daniel,

Yes thats correct I can build lighting and save successfully, but when I reopen the level it crashes the engine. I do not have any modifications to the source it is a blueprint based project. I will try the suggestion of creating a new project however this problem only occurs with this map, other maps in the project work fine. I think I remember that I started this project on either 4.13 or 4.14 and updated to 4.15, could that have caused this? If all this fails I will have to start a new map and just build the level again from scratch.

I remember this problem started when I was messing around with lighting scenarios, which might be a bug actually. The project is a vr architectural walkthrough, in which you can swap out room configurations and load in new ones using level streaming. To do this I have to build lighting in my persistent level (which is the problematic map) and then go through each room configuration and build lighting seperately, afterwards turning off all directional and skylights. The problem occured when lighting scenarios were introduced and lighting in the seperate levels looked incorrect until I set the same lighting scenario for every level in the browser, is it possible that this hacky method was made impossible now with lighting scenarios? I will see if I can reproduce this is a new project after the weekend.