Having errors accessing QAST children when invoking perl6 instead of nqp runtime

QASTChildrenBrokenPerl6

use QAST:from<NQP>;

my $qast := QAST::Block.new(
    QAST::IVal.new( :value(42) )
); 
my $comp_unit := QAST::CompUnit.new(
    # Set the HLL.
    :hll('perl6'),

    # This variable contains the outermost QAST::Block of the
    # program.
    $qast,

    # If we run the program as the mainline, then call the top
    # block automatically.
    :main(QAST::Op.new(
        :op('call'),
        QAST::BVal.new( :value($qast) )
    ))
);

# currently broken when invoked via perl6 but works when invoked from nqp
my $thingy := $comp_unit[0];