diff --git a/util/channel.h b/util/channel.h
index 0225482c0..a8a47680a 100644
--- a/util/channel.h
+++ b/util/channel.h
@@ -60,7 +60,7 @@ class channel {
 
  private:
   std::condition_variable cv_;
-  std::mutex lock_;
+  mutable std::mutex lock_;
   std::queue<T> buffer_;
   bool eof_;
 };