diff --git a/xarray/core/pycompat.py b/xarray/core/pycompat.py
index e8aeb2d0651..bc8b61164f1 100644
--- a/xarray/core/pycompat.py
+++ b/xarray/core/pycompat.py
@@ -103,7 +103,7 @@ def is_0d_dask_array(x):
     return is_duck_dask_array(x) and is_scalar(x)
 
 
-def to_numpy(data):
+def to_numpy(data) -> np.ndarray:
     from xarray.core.indexing import ExplicitlyIndexed
     from xarray.core.parallelcompat import get_chunked_array_type