lib/PublicInbox/Thread.pm | 5 ++++-

diff --git a/lib/PublicInbox/Thread.pm b/lib/PublicInbox/Thread.pm
index a2553da70ab4bab16d5bceaf927ec436a84840e5..7dabf2439927ee2a6546b3218d060b109a999dc4 100644
--- a/lib/PublicInbox/Thread.pm
+++ b/lib/PublicInbox/Thread.pm
@@ -7,7 +7,10 @@ package PublicInbox::Thread;
 use strict;
 use warnings;
 use base qw(Mail::Thread);
-sub _container_class { 'PublicInbox::Thread::Container' }
+
+if ($Mail::Thread::VERSION <= 2.55) {
+	eval q(sub _container_class { 'PublicInbox::Thread::Container' });
+}
 
 package PublicInbox::Thread::Container;
 use strict;