diff --git a/kittens/diff/highlight.py b/kittens/diff/highlight.py
index 7272f8984..dd7824285 100644
--- a/kittens/diff/highlight.py
+++ b/kittens/diff/highlight.py
@@ -161,7 +161,7 @@ def highlight_collection(collection: Collection, aliases: Optional[Dict[str, str
                     if p:
                         is_binary = isinstance(data_for_path(p), bytes)
                         if not is_binary:
-                            jobs[executor.submit(highlight_for_diff, p, aliases)] = p
+                            jobs[executor.submit(highlight_for_diff, p, aliases or {})] = p
         for future in concurrent.futures.as_completed(jobs):
             path = jobs[future]
             try:
diff --git a/kittens/diff/render.py b/kittens/diff/render.py
index 6e6cbdf13..4408e128b 100644
--- a/kittens/diff/render.py
+++ b/kittens/diff/render.py
@@ -36,6 +36,8 @@ def images_supported() -> bool:
 
 class Ref:
 
+    __slots__: Tuple[str, ...] = ()
+
     def __setattr__(self, name: str, value: object) -> None:
         raise AttributeError("can't set attribute")