diff --git a/session.go b/session.go
index b843558ce..529caf480 100644
--- a/session.go
+++ b/session.go
@@ -602,17 +602,17 @@ func (s *Session) GetOrOpenStream(id protocol.StreamID) (utils.Stream, error) {
 // The streamsMutex is locked by OpenStream or GetOrOpenStream before calling this function.
 func (s *Session) newStreamImpl(id protocol.StreamID) (*stream, error) {
 	maxAllowedStreams := uint32(protocol.MaxStreamsMultiplier * float32(s.connectionParametersManager.GetMaxStreamsPerConnection()))
-	if s.openStreamsCount >= maxAllowedStreams {
+	if atomic.LoadUint32(&s.openStreamsCount) >= maxAllowedStreams {
 		return nil, qerr.TooManyOpenStreams
 	}
+	if _, ok := s.streams[id]; ok {
+		return nil, fmt.Errorf("Session: stream with ID %d already exists", id)
+	}
 	stream, err := newStream(s, s.connectionParametersManager, s.flowController, id)
 	if err != nil {
 		return nil, err
 	}
-	if s.streams[id] != nil {
-		return nil, fmt.Errorf("Session: stream with ID %d already exists", id)
-	}
-	s.openStreamsCount++
+	atomic.AddUint32(&s.openStreamsCount, 1)
 	s.streams[id] = stream
 	return stream, nil
 }
@@ -633,7 +633,7 @@ func (s *Session) garbageCollectStreams() {
 			s.windowUpdateManager.RemoveStream(k)
 		}
 		if v.finished() {
-			s.openStreamsCount--
+			atomic.AddUint32(&s.openStreamsCount, ^uint32(0)) // decrement
 			s.streams[k] = nil
 		}
 	}