getting a Null PMC access, not understanding why

rakudo$ git diff
diff --git a/src/setting/Any-str.pm b/src/setting/Any-str.pm
index be0082b..66348a1 100644
--- a/src/setting/Any-str.pm
+++ b/src/setting/Any-str.pm
@@ -111,4 +111,10 @@ sub unpack($template, $target) {
     }
 }
 
+# TODO: '$filename as Str' once support for that is in place
+multi sub lines($filename) {
+    my $filehandle = open($filename, :r);
+    return lines($filehandle);
+}
+
 # vim: ft=perl6
rakudo$ # ...compile
[...]
Linked: perl6
rakudo$ ./perl6 -e 'say lines("~/gwork/november/README")'
Null PMC access in find_method()
current instr.: '_block14' pc 66 (EVAL_16:40)
called from Sub '!UNIT_START' pc 17593 (src/builtins/guts.pir:358)
called from Sub 'parrot;PCT;HLLCompiler;eval' pc 950 (src/PCT/HLLCompiler.pir:527)
called from Sub 'parrot;PCT;HLLCompiler;command_line' pc 1510 (src/PCT/HLLCompiler.pir:798)
called from Sub 'parrot;Perl6;Compiler;main' pc 21977 (perl6.pir:164)
rakudo$