Problem with setting ASTs in Perl 6 grammars

Foo.pl

grammar Foo {
    token element {
        | 'a' <element> { make 'a' ~ $<element>.ast }
        | ';'           { make ';' }
    }
}

my $m = Foo.parse("aaaa;", :rule('element'));
say $m.gist;
say $m.ast;