diff --git a/features/filesystem/sd/SDBlockDevice.h b/features/filesystem/sd/SDBlockDevice.h
index 9d93e6c873..d1972a8f80 100644
--- a/features/filesystem/sd/SDBlockDevice.h
+++ b/features/filesystem/sd/SDBlockDevice.h
@@ -149,7 +149,7 @@ private:
     unsigned _block_size;
     bool _is_initialized;
     bool _dbg;
-    Mutex _lock;
+    mutable Mutex _lock;
 };