diff --git a/Makefile.am b/Makefile.am
index e5bb9db385..e417af5d9b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1378,6 +1378,7 @@ MIXER_API_SRC = \
 libmixer_plugins_a_SOURCES = \
 	src/mixer/plugins/NullMixerPlugin.cxx \
 	src/mixer/plugins/SoftwareMixerPlugin.cxx \
+	src/mixer/plugins/SoftwareMixerPlugin.hxx \
 	src/mixer/plugins/volume_mapping.hxx \
 	src/mixer/plugins/volume_mapping.cxx