Nested map access in RHS fails in exec-model

package com.example.reproducer
import org.drools.modelcompiler.MvelDialectTest.Bus;
dialect "mvel"
global java.util.List result;
rule "rule_mt_1a"
    when
        $bus : Bus( $title: "POWER PLANT" )
    then
        result.add($bus.karaoke.dvd[$title].artist);
end