Custom StepMonitor doesn't work in version 4.x

I created a custom StepMonitor:

private StepMonitor stp = new NullStepMonitor(){
public void performing(String step, boolean dryRun){
System.err.println("DEBUG - Monitor. Step = " + step);
}
};

and I use it in my configuration:

return new MostUsefulConfiguration()
................
.useStepMonitor(stp);

On version 3,9,5 it works as expected, the result is:

Scenario: A scenario with some pending steps
DEBUG - Monitor. Step = Given I am a pending step
Given I am a pending step
DEBUG - Monitor. Step = And I am still pending step
And I am still pending step
DEBUG - Monitor. Step = When a good soul will implement me
When a good soul will implement me
DEBUG - Monitor. Step = Then I shall be happy
Then I shall be happy

But on version 4.0.4 it doesn't work, I have got:

Scenario: A scenario with some pending steps
Given I am a pending step
And I am still pending step
When a good soul will implement me
Then I shall be happy

Here is simple example on GitHub: https://github.com/kordirko/jbehave_stepmonitor.git
Master branch is for version 4.0.4, and branch "Jbehave_3_9_5" is for version 3.9.5