It seems that Gradle's custom serialization doesn't support the List.of() stuff introduced in recent JDKs....

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/FileSignature.java

@@ -126,7 +126,7 @@ public static Promised promise(Iterable<File> files) {
 	}
 
 	public static Promised promise(File file) {
-		return new Promised(List.of(file), null);
+		return new Promised(MoreIterables.toNullHostileList(List.of(file)), null);
 	}
 
 	/** Returns all of the files in this signature, throwing an exception if there are more or less than 1 file. */