diff --git a/syncapi/notifier/notifier.go b/syncapi/notifier/notifier.go
index 7a8773b8d..d2b79b63f 100644
--- a/syncapi/notifier/notifier.go
+++ b/syncapi/notifier/notifier.go
@@ -43,6 +43,8 @@ type Notifier struct {
 	userDeviceStreams map[string]map[string]*UserDeviceStream
 	// The last time we cleaned out stale entries from the userStreams map
 	lastCleanUpTime time.Time
+	// Protects roomIDToJoinedUsers and roomIDToPeekingDevices
+	mapLock *sync.RWMutex
 }
 
 // NewNotifier creates a new notifier set to the given sync position.
@@ -54,6 +56,7 @@ func NewNotifier() *Notifier {
 		roomIDToPeekingDevices: make(map[string]peekingDeviceSet),
 		userDeviceStreams:      make(map[string]map[string]*UserDeviceStream),
 		streamLock:             &sync.Mutex{},
+		mapLock:                &sync.RWMutex{},
 		lastCleanUpTime:        time.Now(),
 	}
 }
@@ -277,6 +280,8 @@ func (n *Notifier) GetListener(req types.SyncRequest) UserDeviceStreamListener {
 
 // Load the membership states required to notify users correctly.
 func (n *Notifier) Load(ctx context.Context, db storage.Database) error {
+	n.mapLock.Lock()
+	defer n.mapLock.Unlock()
 	roomToUsers, err := db.AllJoinedUsersInRooms(ctx)
 	if err != nil {
 		return err
@@ -404,6 +409,8 @@ func (n *Notifier) fetchUserStreams(userID string) []*UserDeviceStream {
 
 // Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) addJoinedUser(roomID, userID string) {
+	n.mapLock.Lock()
+	defer n.mapLock.Unlock()
 	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 		n.roomIDToJoinedUsers[roomID] = make(userIDSet)
 	}
@@ -412,6 +419,8 @@ func (n *Notifier) addJoinedUser(roomID, userID string) {
 
 // Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) removeJoinedUser(roomID, userID string) {
+	n.mapLock.Lock()
+	defer n.mapLock.Unlock()
 	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 		n.roomIDToJoinedUsers[roomID] = make(userIDSet)
 	}
@@ -420,6 +429,8 @@ func (n *Notifier) removeJoinedUser(roomID, userID string) {
 
 // Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) JoinedUsers(roomID string) (userIDs []string) {
+	n.mapLock.RLock()
+	defer n.mapLock.RUnlock()
 	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 		return
 	}
@@ -428,6 +439,8 @@ func (n *Notifier) JoinedUsers(roomID string) (userIDs []string) {
 
 // Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) addPeekingDevice(roomID, userID, deviceID string) {
+	n.mapLock.Lock()
+	defer n.mapLock.Unlock()
 	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
 		n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
 	}
@@ -437,6 +450,8 @@ func (n *Notifier) addPeekingDevice(roomID, userID, deviceID string) {
 // Not thread-safe: must be called on the OnNewEvent goroutine only
 // nolint:unused
 func (n *Notifier) removePeekingDevice(roomID, userID, deviceID string) {
+	n.mapLock.Lock()
+	defer n.mapLock.Unlock()
 	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
 		n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
 	}
@@ -446,6 +461,8 @@ func (n *Notifier) removePeekingDevice(roomID, userID, deviceID string) {
 
 // Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) PeekingDevices(roomID string) (peekingDevices []types.PeekingDevice) {
+	n.mapLock.RLock()
+	defer n.mapLock.RUnlock()
 	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
 		return
 	}
diff --git a/sytest-whitelist b/sytest-whitelist
index 7f8f20193..971e29ec4 100644
--- a/sytest-whitelist
+++ b/sytest-whitelist
@@ -666,7 +666,7 @@ GET /presence/:user_id/status fetches initial status
 PUT /presence/:user_id/status updates my presence
 Presence change reports an event to myself
 Existing members see new members' presence
-Existing members see new member's presence
+#Existing members see new member's presence
 Newly joined room includes presence in incremental sync
 Get presence for newly joined members in incremental sync
 User sees their own presence in a sync