diff --git a/src/prompt_toolkit/layout/screen.py b/src/prompt_toolkit/layout/screen.py
index 0f19f52a8..475f540d1 100644
--- a/src/prompt_toolkit/layout/screen.py
+++ b/src/prompt_toolkit/layout/screen.py
@@ -169,7 +169,7 @@ def __init__(
 
         #: Escape sequences to be injected.
         self.zero_width_escapes: defaultdict[int, defaultdict[int, str]] = defaultdict(
-            lambda: defaultdict(lambda: "")
+            lambda: defaultdict(str)
         )
 
         #: Position of the cursor.