diff --git a/src/output/Thread.cxx b/src/output/Thread.cxx
index fd6ee3ef3..2aee896e1 100644
--- a/src/output/Thread.cxx
+++ b/src/output/Thread.cxx
@@ -207,7 +207,7 @@ AudioOutputControl::WaitForDelay(std::unique_lock<Mutex> &lock) noexcept
 	}
 }
 
-bool
+inline bool
 AudioOutputControl::FillSourceOrClose() noexcept
 try {
 	return source.Fill(mutex);