diff --git a/CHANGES.md b/CHANGES.md
index 1022c2d478..0f7f5cfb55 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -18,6 +18,7 @@ Fixed:
 - Make sure reconnection errors are router through the regulat `on_error` callback in `output.icecast` (#3635)
 - Fixed discontinuity count after a restart in HLS outputs.
 - Fixed memory leaks when using dynamically created sources (`input.harbor`, `input.ffmepg`, SRT sources and `request.dynamic`)
+- Fixed invalid array fill in `add` (#3678)
 
 ---
 
diff --git a/src/core/operators/add.ml b/src/core/operators/add.ml
index 64d4e92eb1..b6eb03a7eb 100644
--- a/src/core/operators/add.ml
+++ b/src/core/operators/add.ml
@@ -147,7 +147,7 @@ class audio_add ~renorm ~power ~field tracks =
       assert (offset <= pos);
       let audio_offset = Frame.audio_of_main offset in
       let pcm = Content.Audio.get_data (Frame.get buf field) in
-      Audio.clear pcm audio_offset (Audio.length pcm);
+      Audio.clear pcm audio_offset (Audio.length pcm - audio_offset);
       List.iter
         (fun { source; fields } ->
           let tmp = self#track_frame source in