diff --git a/xarray/core/ops.py b/xarray/core/ops.py
index 2ed3f81d185..3f08a30e90d 100644
--- a/xarray/core/ops.py
+++ b/xarray/core/ops.py
@@ -42,6 +42,8 @@
 # wrapped in an Variable/DataArray
 NUMPY_UNARY_METHODS = ['astype', 'argsort', 'clip', 'conj', 'conjugate']
 PANDAS_UNARY_FUNCTIONS = ['isnull', 'notnull']
+if pd.__version__ >= "0.21":
+    PANDAS_UNARY_FUNCTIONS.extend(['isna', 'notna'])
 # methods which remove an axis
 REDUCE_METHODS = ['all', 'any']
 NAN_REDUCE_METHODS = ['argmax', 'argmin', 'max', 'min', 'mean', 'prod', 'sum',