diff --git a/src/output/Interface.hxx b/src/output/Interface.hxx
index 5221d328a..565c31b77 100644
--- a/src/output/Interface.hxx
+++ b/src/output/Interface.hxx
@@ -159,6 +159,8 @@ public:
 
 	/**
 	 * Wait until the device has finished playing.
+	 *
+	 * Throws on error.
 	 */
 	virtual void Drain() {}