diff --git a/syncapi/sync/notifier.go b/syncapi/sync/notifier.go
index 30ac3a2e..14bc2efb 100644
--- a/syncapi/sync/notifier.go
+++ b/syncapi/sync/notifier.go
@@ -185,6 +185,7 @@ func (n *Notifier) wakeupUsers(userIDs []string, newPos types.SyncPosition) {
 // fetchUserStream retrieves a stream unique to the given user. If makeIfNotExists is true,
 // a stream will be made for this user if one doesn't exist and it will be returned. This
 // function does not wait for data to be available on the stream.
+// NB: Callers should have locked the mutex before calling this function.
 func (n *Notifier) fetchUserStream(userID string, makeIfNotExists bool) *UserStream {
 	stream, ok := n.userStreams[userID]
 	if !ok && makeIfNotExists {
diff --git a/syncapi/sync/notifier_test.go b/syncapi/sync/notifier_test.go
index 904315e9..808e07cc 100644
--- a/syncapi/sync/notifier_test.go
+++ b/syncapi/sync/notifier_test.go
@@ -143,7 +143,7 @@ func TestNewEventAndJoinedToRoom(t *testing.T) {
 		wg.Done()
 	}()
 
-	stream := n.fetchUserStream(bob, true)
+	stream := lockedFetchUserStream(n, bob)
 	waitForBlocking(stream, 1)
 
 	n.OnNewEvent(&randomMessageEvent, "", nil, syncPositionAfter)
@@ -171,7 +171,7 @@ func TestNewInviteEventForUser(t *testing.T) {
 		wg.Done()
 	}()
 
-	stream := n.fetchUserStream(bob, true)
+	stream := lockedFetchUserStream(n, bob)
 	waitForBlocking(stream, 1)
 
 	n.OnNewEvent(&aliceInviteBobEvent, "", nil, syncPositionAfter)
@@ -199,7 +199,7 @@ func TestEDUWakeup(t *testing.T) {
 		wg.Done()
 	}()
 
-	stream := n.fetchUserStream(bob, true)
+	stream := lockedFetchUserStream(n, bob)
 	waitForBlocking(stream, 1)
 
 	n.OnNewEvent(&aliceInviteBobEvent, "", nil, syncPositionNewEDU)
@@ -230,7 +230,7 @@ func TestMultipleRequestWakeup(t *testing.T) {
 	go poll()
 	go poll()
 
-	stream := n.fetchUserStream(bob, true)
+	stream := lockedFetchUserStream(n, bob)
 	waitForBlocking(stream, 3)
 
 	n.OnNewEvent(&randomMessageEvent, "", nil, syncPositionAfter)
@@ -266,14 +266,14 @@ func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 		}
 		leaveWG.Done()
 	}()
-	bobStream := n.fetchUserStream(bob, true)
+	bobStream := lockedFetchUserStream(n, bob)
 	waitForBlocking(bobStream, 1)
 	n.OnNewEvent(&bobLeaveEvent, "", nil, syncPositionAfter)
 	leaveWG.Wait()
 
 	// send an event into the room. Make sure alice gets it. Bob should not.
 	var aliceWG sync.WaitGroup
-	aliceStream := n.fetchUserStream(alice, true)
+	aliceStream := lockedFetchUserStream(n, alice)
 	aliceWG.Add(1)
 	go func() {
 		pos, err := waitForEvents(n, newTestSyncRequest(alice, syncPositionAfter))
@@ -328,6 +328,15 @@ func waitForBlocking(s *UserStream, numBlocking uint) {
 	}
 }
 
+// lockedFetchUserStream invokes Notifier.fetchUserStream, respecting Notifier.streamLock.
+// A new stream is made if it doesn't exist already.
+func lockedFetchUserStream(n *Notifier, userID string) *UserStream {
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
+
+	return n.fetchUserStream(userID, true)
+}
+
 func newTestSyncRequest(userID string, since types.SyncPosition) syncRequest {
 	return syncRequest{
 		device:        authtypes.Device{UserID: userID},