diff --git a/src/UI/Timeline/AnimationTimeline.gd b/src/UI/Timeline/AnimationTimeline.gd
index 6380c2b62..b8fa97230 100644
--- a/src/UI/Timeline/AnimationTimeline.gd
+++ b/src/UI/Timeline/AnimationTimeline.gd
@@ -543,7 +543,8 @@ func _on_MergeDownLayer_pressed() -> void:
 
 
 func _on_OpacitySlider_value_changed(value) -> void:
-	Global.current_project.frames[Global.current_project.current_frame].cels[Global.current_project.current_layer].opacity = value / 100
+	var cel : Cel = Global.current_project.frames[Global.current_project.current_frame].cels[Global.current_project.current_layer]
+	cel.opacity = value / 100
 	Global.layer_opacity_slider.value = value
 	Global.layer_opacity_slider.value = value
 	Global.layer_opacity_spinbox.value = value