diff --git a/narwhals/_arrow/group_by.py b/narwhals/_arrow/group_by.py
index f232bcd56..117616b00 100644
--- a/narwhals/_arrow/group_by.py
+++ b/narwhals/_arrow/group_by.py
@@ -72,13 +72,13 @@ def agg_arrow(
         simple_aggregations: dict[str, tuple[str, str]] = {}
         for expr in exprs:
             # e.g. agg(nw.mean('a')) # noqa: ERA001
-            if expr._depth != 1:
+            if expr._depth != 1:  # pragma: no cover
                 msg = f"Expr._depth should be 1, found {expr._depth}"
                 raise ValueError(msg)
-            if expr._root_names is None:
+            if expr._root_names is None:  # pragma: no cover
                 msg = "`Expr._root_names` cannot be None"
                 raise ValueError(msg)
-            if expr._output_names is None:
+            if expr._output_names is None:  # pragma: no cover
                 msg = "`Expr._output_names` cannot be None"
                 raise ValueError(msg)
 
diff --git a/narwhals/_expression_parsing.py b/narwhals/_expression_parsing.py
index deb3a5a79..24855d4d9 100644
--- a/narwhals/_expression_parsing.py
+++ b/narwhals/_expression_parsing.py
@@ -191,7 +191,7 @@ def func(df: CompliantDataFrame) -> list[CompliantSeries]:
                 out.append(_out)
         if expr._output_names is not None and (
             [s.name for s in out] != expr._output_names
-        ):
+        ):  # pragma: no cover
             msg = "Found invalid series name"
             raise ValueError(msg)
         return out
@@ -217,7 +217,7 @@ def func(df: CompliantDataFrame) -> list[CompliantSeries]:
     if not (
         (output_names is None and root_names is None)
         or (output_names is not None and root_names is not None)
-    ):
+    ):  # pragma: no cover
         msg = "output_names and root_names are incompatible"
         raise ValueError(msg)
 
diff --git a/narwhals/_pandas_like/group_by.py b/narwhals/_pandas_like/group_by.py
index 8950f64df..0ce54ad48 100644
--- a/narwhals/_pandas_like/group_by.py
+++ b/narwhals/_pandas_like/group_by.py
@@ -131,13 +131,13 @@ def agg_pandas(  # noqa: PLR0915
                 continue
 
             # e.g. agg(nw.mean('a')) # noqa: ERA001
-            if expr._depth != 1:
+            if expr._depth != 1:  # pragma: no cover
                 msg = f"Expr._depth should be 1, found {expr._depth}"
                 raise ValueError(msg)
-            if expr._root_names is None:
+            if expr._root_names is None:  # pragma: no cover
                 msg = "`Expr._root_names` cannot be None"
                 raise ValueError(msg)
-            if expr._output_names is None:
+            if expr._output_names is None:  # pragma: no cover
                 msg = "`Expr._output_names` cannot be None"
                 raise ValueError(msg)