Method "" in traits class has illegal signature "(Ljava/lang/Class;)V"

[Schalk Cronje]

I have a library compiled with JDK8 which contains some traits of which 
the gist one of them is

   @CompileStatic
   trait LocalStackFixture {
   }
It then gets used by something like

   class LocalStackFixtureSpec extends Specification implements
   LocalStackFixture {
   }

When I recompile the project with JDK11 I get

    Method "<clinit>" in class 
testfixtures/localstack/LocalStackFixture$Trait$Helper has illegal 
signature "(Ljava/lang/Class;)V"

This is Groovy 2.5.4.

I don't really have a clue where to start looking.

--
Schalk W. Cronjé
Twitter / Ello: @ysb33r