diff --git a/xarray/coding/cftime_offsets.py b/xarray/coding/cftime_offsets.py
index 90478dffe2d..9dbc60ef0f3 100644
--- a/xarray/coding/cftime_offsets.py
+++ b/xarray/coding/cftime_offsets.py
@@ -112,8 +112,8 @@ def get_date_type(calendar, use_cftime=True):
 
 
 class BaseCFTimeOffset:
-    _freq: ClassVar[str]
-    _day_option: ClassVar[DayOption]
+    _freq: ClassVar[str | None] = None
+    _day_option: ClassVar[DayOption | None] = None
     n: int
 
     def __init__(self, n: int = 1) -> None:
@@ -124,7 +124,7 @@ def __init__(self, n: int = 1) -> None:
             )
         self.n = n
 
-    def rule_code(self) -> str:
+    def rule_code(self) -> str | None:
         return self._freq
 
     def __eq__(self, other: object) -> bool:
diff --git a/xarray/tests/test_cftime_offsets.py b/xarray/tests/test_cftime_offsets.py
index eabb7d2f4d6..78aa49c7f83 100644
--- a/xarray/tests/test_cftime_offsets.py
+++ b/xarray/tests/test_cftime_offsets.py
@@ -511,7 +511,7 @@ def test_Microsecond_multiplied_float_error():
     ],
     ids=_id_func,
 )
-def test_neg(offset, expected):
+def test_neg(offset: BaseCFTimeOffset, expected: BaseCFTimeOffset) -> None:
     assert -offset == expected