diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 5f69b3deeab4e..a18db1867e0c1 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -4211,7 +4211,7 @@ def __setitem__(self, key, value) -> None:
                 warn_copy_on_write()
                 or (
                     not warn_copy_on_write()
-                    and any(b.refs.has_reference() for b in self._mgr.blocks)
+                    and any(b.refs.has_reference() for b in self._mgr.blocks)  # type: ignore[union-attr]
                 )
             ):
                 warnings.warn(