Derived Blueprint class doesn't compile its parent

I have some Blueprint classes that derives from each other:
AWheeledVehicle → BP_WheeledVehicleBase → BP_BusBase → BP_MAN_LionsCoach

When I want to compile BP_MAN_LionsCoach I get an Error:

LogBlueprint:Error: [compiler BP_MAN_LionsCoach] Error This blueprint (self) is not a TRASHCLASS_BP_WheeledVehicleBase_185, therefore ' Target ' must have a connection.

When I compile BP_BusBase all is fine and then I can compile BP_MAN_LionsCoach without any Problem. It seems that the Compiler doesn’t compiles the dependent base Blueprint.

This is annoying but until now this wasn’t a big problem. But now I wanted to package my project and while packaging I get the same error. But I cannot compile the base class manually for packaging. So I cannot package my whole project. Is there any way to repair this behaviour?

UE 4.10.2 Launcher Version


Have you converted your project up from a previous version? If so, it is often recommended that you remake your blueprints, as sometimes converting the project to a new version can cause this sort of error to appear.

Yes this project was converted from 4.9.

This behavior is very frustrating. Our game massivly uses blueprints and we cannot rebuild our whole game with every engine update. There must be a reason for this bug and this should be fixed. I think a version update is an elemetary feature of the engine.

You could try right-clicking on the blueprint node(s) and selecting the refresh nodes option.

Converting projects to different engine versions often entail more work than simply copying the project over. Re-working blueprints and updating code to meet with any changes made in the newer versions are to be expected.

However, the issue you are experiencing seems to be unintended, but I have not been able to reproduce the same issue on my end after converting a project through multiple engine versions. Do you have any solid repro for this issue?

With a lot of work I’ve rebuild some Blueprints (included the 3 one above) but I get the same error. I’ve mirgrated and reduced my blueprints to a new Project. Maybe you can find the problem. You only have to click on the compile button of BP_MAN_LionsCoach and you see the errors.

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I was able to get the compile errors to go away by connecting the execution pin to the Print Text node in the BP_GlobalFunctions, and then going through and compiling the WheeledVehicleBase, BusBase, and then Lions Coach blueprints. Doing this, I received no more compiler errors in the blueprints, even upon re-opening the project.

Give that a shot and let me know if it fixes the errors you are seeing.

In which function you added a Print Text Node?

It was already in GlobalFunctions, specifically in the Set Switch at Switch Material function, and I just reconnected the execution pin.

This doesn’t work for me (even in my Test-Project). Every time I restart the Editor I get the error again.

So if you compile the Wheeled Vehicle Base blueprint, and then compile the BP_MAN_LionsCoach, you still see compile errors? For me, once I compiled the Wheeled Vehicle Base, LionsCoach was resolved of all compiler errors.

Yes thats correct but after restarting the editor the error appears again. And it is not possible to package the project.

Looking at the project, it seems that a lot of your blueprints are referencing functions that don’t seem to exist where they are supposed to. For one example, your Game Mode is calling a function called Init Bus Stops, which doesn’t seem to exist in the Game Instance where it is attempting to call it from. Did you happen to remove some functions when you were recreating your blueprints?

I think I have overlooked these calls when reducing the blueprints for a sample. In my original project they are all correct. I think they are independent to the bug.

To be on the safe side I removed all wrong calls in the Blueprints but that doesn’t affects the errors.

For testing I converted my project to 4.11 P3. There I got no errors. Is there any change in 4.11 that fixes this behaviour?

When you say you reduced the blueprints, what do you mean by that? Did you not create the blueprints from scratch in the clean project?

If you just migrated the blueprints over, this could be why issues are still remaining. What you’ll need to try in this case is completely remaking the blueprints without copying any existing nodes.

I already recreated a lot of Blueprints in my Project but It’s impossible to recreate my whole project. This would cost weeks.

After some investigation, I wasn’t able to find a particular bug report that was related to this issue. However, it is possible that there was a fix implemented, or that fixing another issue also resolved the issue that you were experiencing.

I am glad to hear that converting the project to 4.11 fixed your issue. As such, I will be marking this post as resolved.

Have a great day