Failed to create block entity DUMMY (path of location: minecraft:DUMMY)

I created a new world (superflat with Roofed Forest biome) and flew around the map generating new terrain. A few of these errors appeared in the game log:

12:07:17	bey	Failed to create block entity DUMMY
i: Non [a-z0-9/._-] character in path of location: minecraft:DUMMY
	at oe.<init>(SourceFile:36)
	at oe.<init>(SourceFile:41)
	at bez.a(SourceFile:63)
	at bey.c(SourceFile:72)
	at bim.a(SourceFile:526)
	at bim.a(SourceFile:97)
	at sd.g(SourceFile:93)
	at sd.d(SourceFile:146)
	at bnt.a(SourceFile:225)
	at bnt.a(SourceFile:78)
	at bpv.a(SourceFile:16)
	at bpv.a(SourceFile:13)
	at bkd.a(SourceFile:27)
	at auw.a(SourceFile:469)
	at bjg.a(SourceFile:144)
	at sr.a(SourceFile:12)
	at sq.a(SourceFile:33)
	at bhy.a(SourceFile:85)
	at sw.a(SourceFile:54)
	at sw.a(SourceFile:23)
	at zt$a.a(SourceFile:143)
	at zt$a$$Lambda$891/13133174.apply(Unknown Source)
	at java.util.concurrent.CompletableFuture$AsyncApply.exec(CompletableFuture.java:501)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:902)
	at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1689)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1644)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)