Kernel change broken wireguard builds in unexpected way

Normally when a kernel change causes a problem there is a compile error, but I'm seeing a make problem. Builds work with kernel-4.16.0-0.rc6.git0.2.fc28.x86_64, but not with kernel-4.16.0-0.rc6.git2.2.fc29.x86_64. The error output is: [bruno@cerberus src]$ make KERNELDIR=/lib/modules/4.16.0-0.rc6.git2.2.fc29.x86_64/build clean all CLEAN /home/bruno/WireGuard/src/.tmp_versions CLEAN /home/bruno/WireGuard/src/Module.symvers CLEAN /home/bruno/WireGuard/src/tools/{wg,*.o,*.d} make[2]: *** No rule to make target '/home/bruno/WireGuard/src/main.o', needed by '/home/bruno/WireGuard/src/wireguard.o'. Stop. make[1]: *** [Makefile:1556: _module_/home/bruno/WireGuard/src] Error 2 make: *** [Makefile:36: module] Error 2 Is this something Fedora specific? I wouldn't expect an rc6 change to cause an issue like this. I'm going to try building an upstream kernel to see if the problem is upstream, but I probably won't have an answer until tomorrow.