diff --git a/include/ircd/m/room/auth.h b/include/ircd/m/room/auth.h
index 48f2177f0..d89952bc5 100644
--- a/include/ircd/m/room/auth.h
+++ b/include/ircd/m/room/auth.h
@@ -27,7 +27,7 @@ struct ircd::m::room::auth
 
 	static bool is_power_event(const event &);
 	static std::array<event::idx, 5> relative_idx(const event &, const room &);
-	static std::array<event::idx, 4> static_idx(const event &);
+	static std::array<event::idx, 5> static_idx(const event &);
 
 	static passfail check(const event &, hookdata &);
 	static passfail check(const event &, const vector_view<event::idx> &);
diff --git a/matrix/room_auth.cc b/matrix/room_auth.cc
index 7b1340407..be699d3ac 100644
--- a/matrix/room_auth.cc
+++ b/matrix/room_auth.cc
@@ -553,7 +553,7 @@ ircd::m::check_room_auth_rule_9(const m::event &event,
 			};
 }
 
-std::array<ircd::m::event::idx, 4>
+std::array<ircd::m::event::idx, 5>
 ircd::m::room::auth::static_idx(const event &event)
 {
 	const m::event::prev refs
@@ -580,6 +580,7 @@ ircd::m::room::auth::static_idx(const event &event)
 		count > 1? m::index(refs.auth_event(1)): 0UL,
 		count > 2? m::index(refs.auth_event(2)): 0UL,
 		count > 3? m::index(refs.auth_event(3)): 0UL,
+		count > 4? m::index(refs.auth_event(4)): 0UL,
 	};
 }