diff --git a/aos/common/mutex.h b/aos/common/mutex.h
index 4c06aae..b6b277c 100644
--- a/aos/common/mutex.h
+++ b/aos/common/mutex.h
@@ -11,6 +11,8 @@
 
 namespace aos {
 
+class Condition;
+
 // An abstraction of a mutex that has implementations both for the
 // atom and for the cRIO.
 // If there are multiple tasks or processes contending for the mutex,
@@ -42,6 +44,9 @@
   typedef mutex ImplementationType;
 #endif
   ImplementationType impl_;
+
+  friend class Condition;  // for access to impl_
+
 #ifdef __VXWORKS__
   DISALLOW_COPY_AND_ASSIGN(Mutex);
 #endif