diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index b48eaf78..1a121987 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -329,6 +329,12 @@ func SendJoin(
 			JSON: jsonerror.NotFound("Room does not exist"),
 		}
 	}
+	if !stateAndAuthChainResponse.StateKnown {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: jsonerror.Forbidden("State not known"),
+		}
+	}
 
 	// Check if the user is already in the room. If they're already in then
 	// there isn't much point in sending another join event into the room.
diff --git a/federationapi/routing/state.go b/federationapi/routing/state.go
index 6fdce20c..5377eb88 100644
--- a/federationapi/routing/state.go
+++ b/federationapi/routing/state.go
@@ -135,6 +135,12 @@ func getState(
 		return nil, nil, &resErr
 	}
 
+	if !response.StateKnown {
+		return nil, nil, &util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound("State not known"),
+		}
+	}
 	if response.IsRejected {
 		return nil, nil, &util.JSONResponse{
 			Code: http.StatusNotFound,
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index c8e6f9dc..32d63bb5 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -227,6 +227,7 @@ type QueryStateAndAuthChainResponse struct {
 	// Do all the previous events exist on this roomserver?
 	// If some of previous events do not exist this will be false and StateEvents will be empty.
 	PrevEventsExist bool `json:"prev_events_exist"`
+	StateKnown      bool `json:"state_known"`
 	// The state and auth chain events that were requested.
 	// The lists will be in an arbitrary order.
 	StateEvents     []*gomatrixserverlib.HeaderedEvent `json:"state_events"`
diff --git a/roomserver/internal/query/query.go b/roomserver/internal/query/query.go
index f5d8c2d4..6dce2bc3 100644
--- a/roomserver/internal/query/query.go
+++ b/roomserver/internal/query/query.go
@@ -503,10 +503,11 @@ func (r *Queryer) QueryStateAndAuthChain(
 	}
 
 	var stateEvents []*gomatrixserverlib.Event
-	stateEvents, rejected, err := r.loadStateAtEventIDs(ctx, info, request.PrevEventIDs)
+	stateEvents, rejected, stateMissing, err := r.loadStateAtEventIDs(ctx, info, request.PrevEventIDs)
 	if err != nil {
 		return err
 	}
+	response.StateKnown = !stateMissing
 	response.IsRejected = rejected
 	response.PrevEventsExist = true
 
@@ -542,15 +543,18 @@ func (r *Queryer) QueryStateAndAuthChain(
 	return err
 }
 
-func (r *Queryer) loadStateAtEventIDs(ctx context.Context, roomInfo *types.RoomInfo, eventIDs []string) ([]*gomatrixserverlib.Event, bool, error) {
+// first bool: is rejected, second bool: state missing
+func (r *Queryer) loadStateAtEventIDs(ctx context.Context, roomInfo *types.RoomInfo, eventIDs []string) ([]*gomatrixserverlib.Event, bool, bool, error) {
 	roomState := state.NewStateResolution(r.DB, roomInfo)
 	prevStates, err := r.DB.StateAtEventIDs(ctx, eventIDs)
 	if err != nil {
 		switch err.(type) {
 		case types.MissingEventError:
-			return nil, false, nil
+			return nil, false, true, nil
+		case types.MissingStateError:
+			return nil, false, true, nil
 		default:
-			return nil, false, err
+			return nil, false, false, err
 		}
 	}
 	// Currently only used on /state and /state_ids
@@ -567,12 +571,11 @@ func (r *Queryer) loadStateAtEventIDs(ctx context.Context, roomInfo *types.RoomI
 		ctx, prevStates,
 	)
 	if err != nil {
-		return nil, rejected, err
+		return nil, rejected, false, err
 	}
 
 	events, err := helpers.LoadStateEvents(ctx, r.DB, stateEntries)
-
-	return events, rejected, err
+	return events, rejected, false, err
 }
 
 type eventsFromIDs func(context.Context, []string) ([]types.Event, error)