Forge build fails with Index 0 out of bounds for length 0

[BlayTheNinth]

When using a fresh clone of the multi loader template, ForgeGradle is running into an odd exception in reobfJar during build. I've tried experimenting with removing certain parts of the template, but couldn't pinpoint what exactly is causing the issue.

org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':forge:reobfJar'.
        ...
Caused by: java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
        ...
        at net.minecraftforge.srgutils.InternalUtils.loadNamed(InternalUtils.java:39)
        at net.minecraftforge.srgutils.InternalUtils.load(InternalUtils.java:28)
        at net.minecraftforge.srgutils.IMappingFile.load(IMappingFile.java:27)
        at net.minecraftforge.srgutils.IMappingFile.load(IMappingFile.java:22)
        at net.minecraftforge.gradle.userdev.tasks.RenameJarInPlace.apply(RenameJarInPlace.java:66)

Full Stacktrace

Also reported at MinecraftForge/ForgeGradle#937