diff --git a/src/output/plugins/WinmmOutputPlugin.hxx b/src/output/plugins/WinmmOutputPlugin.hxx
index 5e29121b8..bf8a2e34a 100644
--- a/src/output/plugins/WinmmOutputPlugin.hxx
+++ b/src/output/plugins/WinmmOutputPlugin.hxx
@@ -29,7 +29,7 @@
 #include <windef.h>
 #include <mmsystem.h>
 
-struct WinmmOutput;
+class WinmmOutput;
 
 extern const struct AudioOutputPlugin winmm_output_plugin;