[MC-173446] NBT to SNBT converter fails: "String index out of range: -4"

If the tutorial in the German Minecraft wiki is correct (the English one doesn't really have a tutorial, just a general description), then the data generator is broken.
I made a copy of "level.dat" named "level.nbt" and a copy of "r.-1.-1.mca" named "r.-1.-1.nbt", both result in the same error when I try to convert them:
[fabian@laptop Downloads]$ java -cp server.jar net.minecraft.data.Main --dev --input "/home/fabian/hdd/drive/minecraft/saves/test/level.nbt" --output "/home/fabian/Downloads/test"
[16:48:25] [main/INFO]: Starting provider: NBT to SNBT
Exception in thread "main" [16:48:25] [main/INFO]: [STDERR]: java.lang.StringIndexOutOfBoundsException: String index out of range: -4
[16:48:25] [main/INFO]: [STDERR]: 	at java.lang.String.substring(String.java:1967)
[16:48:25] [main/INFO]: [STDERR]: 	at ic.a(SourceFile:43)
[16:48:25] [main/INFO]: [STDERR]: 	at ic.a(SourceFile:32)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.Iterator.forEachRemaining(Iterator.java:116)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
[16:48:25] [main/INFO]: [STDERR]: 	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:485)
[16:48:25] [main/INFO]: [STDERR]: 	at ic.a(SourceFile:32)
[16:48:25] [main/INFO]: [STDERR]: 	at gy.c(SourceFile:48)
[16:48:25] [main/INFO]: [STDERR]: 	at net.minecraft.data.Main.main(SourceFile:55)
[fabian@laptop Downloads]$ java -cp server.jar net.minecraft.data.Main --dev --input "/home/fabian/hdd/drive/minecraft/saves/test/region/r.-1.-1.nbt" --output "/home/fabian/Downloads/test"
[16:49:41] [main/INFO]: Starting provider: NBT to SNBT
Exception in thread "main" [16:49:41] [main/INFO]: [STDERR]: java.lang.StringIndexOutOfBoundsException: String index out of range: -4
[16:49:41] [main/INFO]: [STDERR]: 	at java.lang.String.substring(String.java:1967)
[16:49:41] [main/INFO]: [STDERR]: 	at ic.a(SourceFile:43)
[16:49:41] [main/INFO]: [STDERR]: 	at ic.a(SourceFile:32)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.Iterator.forEachRemaining(Iterator.java:116)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
[16:49:41] [main/INFO]: [STDERR]: 	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:485)
[16:49:41] [main/INFO]: [STDERR]: 	at ic.a(SourceFile:32)
[16:49:41] [main/INFO]: [STDERR]: 	at gy.c(SourceFile:48)
[16:49:41] [main/INFO]: [STDERR]: 	at net.minecraft.data.Main.main(SourceFile:55)