diff --git a/pandas/core/strings/accessor.py b/pandas/core/strings/accessor.py
index 71a50c69bfee1..f8dad3b252223 100644
--- a/pandas/core/strings/accessor.py
+++ b/pandas/core/strings/accessor.py
@@ -562,6 +562,8 @@ def cat(
         if sep is None:
             sep = ""
 
+        data: Series | np.ndarray
+
         if isinstance(self._orig, ABCIndex):
             data = Series(self._orig, index=self._orig, dtype=self._orig.dtype)
         else:  # Series
@@ -569,9 +571,7 @@ def cat(
 
         # concatenate Series/Index with itself if no "others"
         if others is None:
-            # error: Incompatible types in assignment (expression has type
-            # "ndarray", variable has type "Series")
-            data = ensure_object(data)  # type: ignore[assignment]
+            data = ensure_object(data)
             na_mask = isna(data)
             if na_rep is None and na_mask.any():
                 return sep.join(data[~na_mask])