diff --git a/xarray/core/resample.py b/xarray/core/resample.py
index f1776cf3078..ed665ad4048 100644
--- a/xarray/core/resample.py
+++ b/xarray/core/resample.py
@@ -321,6 +321,7 @@ def reduce(
         self,
         func: Callable[..., Any],
         dim: Union[None, Hashable, Sequence[Hashable]] = None,
+        *,
         axis: Union[None, int, Sequence[int]] = None,
         keep_attrs: bool = None,
         keepdims: bool = False,