diff --git a/source/blender/editors/include/ED_outliner.h b/source/blender/editors/include/ED_outliner.h
index d3fc5174dd9..0325ad9fdba 100644
--- a/source/blender/editors/include/ED_outliner.h
+++ b/source/blender/editors/include/ED_outliner.h
@@ -29,6 +29,7 @@ extern "C" {
 
 struct Base;
 struct ListBase;
+struct SpaceOutliner;
 struct bContext;
 
 bool ED_outliner_collections_editor_poll(struct bContext *C);