diff --git a/libavfilter/vf_phase.c b/libavfilter/vf_phase.c
index 2dcc6faa49..281f73636b 100644
--- a/libavfilter/vf_phase.c
+++ b/libavfilter/vf_phase.c
@@ -42,7 +42,7 @@ enum PhaseMode {
 typedef struct PhaseContext {
     const AVClass *class;
     enum PhaseMode mode;
-    AVFrame *frame;
+    AVFrame *frame; /* previous frame */
     int nb_planes;
     int planeheight[4];
     int linesize[4];