NullPointerException caused by DefaultReporterFactory.printTestFailures(DefaultReporterFactory.java:403)

I'm trying to run Kotlin Spek tests (based on JUnit) from Maven. I've followed the guide on their website, and can run the tests locally perfectly fine.

When I run the Maven "test" task however, some tests are annotated with "FAILURE" in the output, with a plain "null" value appearing in the console. Upon running "mvn test -e", I get the attached stack trace.

I have attached my maven pom.xml file as well.