diff --git a/libavfilter/vf_alphamerge.c b/libavfilter/vf_alphamerge.c
index 72b93c780f..85b6d9b61a 100644
--- a/libavfilter/vf_alphamerge.c
+++ b/libavfilter/vf_alphamerge.c
@@ -38,6 +38,8 @@
 enum { Y, U, V, A };
 
 typedef struct AlphaMergeContext {
+    const AVClass *class;
+
     int is_packed_rgb;
     uint8_t rgba_map[4];
     AVFrame *main_frame;