diff --git a/xarray/coding/cftime_offsets.py b/xarray/coding/cftime_offsets.py
index d724554b458..2ee38a20a4d 100644
--- a/xarray/coding/cftime_offsets.py
+++ b/xarray/coding/cftime_offsets.py
@@ -79,7 +79,7 @@ def get_date_type(calendar):
 
 class BaseCFTimeOffset(object):
     _freq = None  # type: ClassVar[str]
-    _day_option = None
+    _day_option = None  # type: ClassVar[str]
 
     def __init__(self, n=1):
         if not isinstance(n, int):