diff --git a/session.go b/session.go
index 2703f283..b086e378 100644
--- a/session.go
+++ b/session.go
@@ -27,6 +27,7 @@ var (
 	errReopeningStreamsNotAllowed  = errors.New("Reopening Streams not allowed")
 	errRstStreamOnInvalidStream    = errors.New("RST_STREAM received for unknown stream")
 	errWindowUpdateOnInvalidStream = errors.New("WINDOW_UPDATE received for unknown stream")
+	errWindowUpdateOnClosedStream  = errors.New("WINDOW_UPDATE received for an already closed stream")
 )
 
 // StreamCallback gets a stream frame and returns a reply frame
@@ -172,6 +173,8 @@ func (s *Session) run() {
 				s.Close(err, true) // TODO: sent correct error code here
 			case errRstStreamOnInvalidStream:
 				utils.Errorf("Ignoring error in session: %s", err.Error())
+			// Can happen when we already sent the last StreamFrame with the FinBit, but the client already sent a WindowUpdate for this Stream
+			case errWindowUpdateOnClosedStream:
 			default:
 				s.Close(err, true)
 			}
@@ -285,12 +288,16 @@ func (s *Session) handleWindowUpdateFrame(frame *frames.WindowUpdateFrame) error
 		return nil
 	}
 	s.streamsMutex.RLock()
-	stream, ok := s.streams[frame.StreamID]
-	s.streamsMutex.RUnlock()
+	defer s.streamsMutex.RUnlock()
 
-	if !ok {
+	stream, streamExists := s.streams[frame.StreamID]
+
+	if !streamExists {
 		return errWindowUpdateOnInvalidStream
 	}
+	if stream == nil {
+		return errWindowUpdateOnClosedStream
+	}
 
 	stream.UpdateSendFlowControlWindow(frame.ByteOffset)
 
diff --git a/session_test.go b/session_test.go
index c2d0ba64..ef2cd5eb 100644
--- a/session_test.go
+++ b/session_test.go
@@ -300,6 +300,15 @@ var _ = Describe("Session", func() {
 			})
 			Expect(err).To(MatchError(errWindowUpdateOnInvalidStream))
 		})
+
+		It("errors when receiving a WindowUpdateFrame for a closed stream", func() {
+			session.streams[5] = nil // this is what the garbageCollectStreams() does when a Stream is closed
+			err := session.handleWindowUpdateFrame(&frames.WindowUpdateFrame{
+				StreamID:   5,
+				ByteOffset: 1337,
+			})
+			Expect(err).To(MatchError(errWindowUpdateOnClosedStream))
+		})
 	})
 
 	Context("closing", func() {