diff --git a/thumb.go b/thumb.go
index b1bd24c..a6d0beb 100644
--- a/thumb.go
+++ b/thumb.go
@@ -62,7 +62,12 @@ func (this *Server) handleThumb(w http.ResponseWriter, r *http.Request, thumbnai
 	}
 
 	// Only a limited number of thumbnails can be generated concurrently
-	<-this.thumbnailSem
+	select {
+	case <-this.thumbnailSem:
+	case <-r.Context().Done():
+		http.Error(w, r.Context().Err().Error(), 400) // probably won't be delivered anyway
+		return
+	}
 	defer func() { this.thumbnailSem <- struct{}{} }()
 
 	if ctx.Err() != nil {