diff --git a/common/os/Mutex.h b/common/os/Mutex.h
index 2a54b20..63c7e0c 100644
--- a/common/os/Mutex.h
+++ b/common/os/Mutex.h
@@ -31,7 +31,7 @@
     void unlock();
 
   private:
-    friend Condition;
+    friend class Condition;
 
     void* systemMutex;
   };