diff --git a/narwhals/pandas_like/group_by.py b/narwhals/pandas_like/group_by.py
index 6876828f2..81761c0d9 100644
--- a/narwhals/pandas_like/group_by.py
+++ b/narwhals/pandas_like/group_by.py
@@ -90,6 +90,7 @@ def agg(
             for key, name in zip(keys, self._keys):
                 out[name].append(key)
         for expr in exprs:
+            assert expr._output_names is not None
             if len(expr._output_names) != 1:
                 msg = (
                     "Multi-output non-elementary aggregations are not supported in group_by.agg.\n"
diff --git a/narwhals/pandas_like/utils.py b/narwhals/pandas_like/utils.py
index 1f111fc49..dbfcd8e16 100644
--- a/narwhals/pandas_like/utils.py
+++ b/narwhals/pandas_like/utils.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from copy import copy
 from typing import TYPE_CHECKING
 from typing import Any
 from typing import Iterable
@@ -202,13 +203,13 @@ def func(df: DataFrame | LazyFrame) -> list[Series]:
                 out.append(plx._create_series_from_scalar(_out, column))
         return out
 
-    root_names = expr._root_names
-    for arg in args:
-        if isinstance(arg, Expr):
-            root_names.extend(arg._root_names)
-    for arg in kwargs.values():
-        if isinstance(arg, Expr):
-            root_names.extend(arg._root_names)
+    root_names = copy(expr._root_names)
+    for arg in list(args) + list(kwargs.values()):
+        if root_names is not None and isinstance(arg, Expr):
+            if arg._root_names is not None:
+                root_names.extend(arg._root_names)
+            else:
+                root_names = None
 
     return plx._create_expr_from_callable(  # type: ignore[return-value]
         func,