UnrealBuildTool fails with null argument

Development Programming & Scripting
1

After upgrading my project to 4.17 it’s started occasionally failing to compile because UnrealBuildTool encounters a System.ArgumentNullException when it tries to run. The full callstack is:

2>EXEC : error : System.ArgumentNullException: Value cannot be null.
2>  Parameter name: source
2>     at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
2>     at UnrealBuildTool.ExternalExecution.ExecuteHeaderToolIfNecessary(BuildConfiguration BuildConfiguration, UEBuildTarget Target, CppCompileEnvironment GlobalCompileEnvironment, List`1 UObjectModules, FileReference ModuleInfoFileName, ECompilationResult& UHTResult)
2>     at UnrealBuildTool.UnrealBuildTool.RunUBT(BuildConfiguration BuildConfiguration, String[] Arguments, FileReference ProjectFile)

I don’t recall encountering this error while testing the preview but that may have simply been luck since I didn’t do more than occasionally make sure all my code still compiled and ran. Doing a full rebuild of my project fixes the error but I’d like to avoid having to do that if possible since it usually takes a few minutes.

2

Further testing finds that this only occurs when I change a header file. I made a new (blank) code project and encounter the exact same issue and tried it on a different machine so it doesn’t seem to be an issue with either my project or computer.

3

I’m also having this issue when changing a header.

4

It’s not a fix, but if you regenerate project files you can compile again.

5

Adding my own voice to this. Added two transient properties to a class header, and was hit by this exact build error. Clean and rebuild got me past it.

6

I’m having the same issue.

The workaround I’ve used is to Rebuild/build clean - works as well.

7

I have the same issue after updating to 4.17.
I can confirm that both workarounds work, but it is quite cumbersome and time consuming to rebuild every time.

Best

8

Have you tried this fix by Ben Marsh from Epic?

I was facing similar issue,this has solved my problem.

I hope that,it helps you.

[Link to forum post by Ben][1]

Annoying compile issue in 4.17 preview 3 - C++ - Epic Developer Community Forums

9

Yep, that did it. Didn’t think to check that part of the forums so thanks for the link.

10

Same error, when changing a .h file with UE 4.18p1.