diff --git a/src/filter/plugins/NormalizeFilterPlugin.hxx b/src/filter/plugins/NormalizeFilterPlugin.hxx
index fb75e47e7..1e3e1f0de 100644
--- a/src/filter/plugins/NormalizeFilterPlugin.hxx
+++ b/src/filter/plugins/NormalizeFilterPlugin.hxx
@@ -23,8 +23,6 @@
 #include <memory>
 
 class PreparedFilter;
-class Filter;
-struct AudioFormat;
 
 std::unique_ptr<PreparedFilter>
 normalize_filter_prepare() noexcept;