diff --git a/src/output/Internal.hxx b/src/output/Internal.hxx
index 4973b0bee..a5b56edda 100644
--- a/src/output/Internal.hxx
+++ b/src/output/Internal.hxx
@@ -238,7 +238,7 @@ struct AudioOutput {
 	/**
 	 * This mutex protects #open, #fail_timer, #pipe.
 	 */
-	Mutex mutex;
+	mutable Mutex mutex;
 
 	/**
 	 * This condition object wakes up the output thread after