SourceKitten skipping class when generating documentation

I'm trying to document a project using SourceKitten.The project only contains a Swift file with a class that inherits from NSObject. With the terminal command:

sourcekitten doc -- -workspace ExampleApp.xcworkspace -scheme ExampleService > swiftDoc-example.json

I get the output:

Running xcodebuild
Checking xcodebuild -showBuildSettings
Assuming New Build System is used

and an empty JSON file. I don't know what the problem is because the class is public and the project can be built with Xcode. The class imports some frameworks, which I was able to document successfully. I have created an example class to see if it's at least documented. I also tried /// and /** */ for the class documentation. Unfortunately without success.

I'm desperate and I don't know what to do anymore.

Reproduce the sourcekitten doc command against ExampleApp.xcworkspace with the ExampleService scheme and inspect why the generated swiftDoc-example.json is empty. Compare the result with the successfully documented imported frameworks; done means the public NSObject-derived class appears in the generated documentation output.