diff --git a/src/cocoa/src/toga_cocoa/window.py b/src/cocoa/src/toga_cocoa/window.py
index 4b6f0232c1..586196ff4a 100644
--- a/src/cocoa/src/toga_cocoa/window.py
+++ b/src/cocoa/src/toga_cocoa/window.py
@@ -168,7 +168,7 @@ def __init__(self, interface, title, position, size):
             defer=False,
         )
         self.native.interface = self.interface
-        self.native._impl = self
+        self.native.impl = self
 
         self.set_title(title)
         self.set_size(size)