diff --git a/dummy/src/toga_dummy/window.py b/dummy/src/toga_dummy/window.py
index 05586cedc8..ca7e3a93ef 100644
--- a/dummy/src/toga_dummy/window.py
+++ b/dummy/src/toga_dummy/window.py
@@ -105,7 +105,8 @@ def get_size(self) -> Size:
     def set_size(self, size):
         if self.get_size() != size:
             self._set_value("size", size)
-            self.interface.on_resize()
+            if self._get_value("size", None) is not None:
+                self.interface.on_resize()
 
     ######################################################################
     # Window position