diff --git a/tensorflow/python/autograph/operators/py_builtins.py b/tensorflow/python/autograph/operators/py_builtins.py
index 439d7d0c285..152a69013cd 100644
--- a/tensorflow/python/autograph/operators/py_builtins.py
+++ b/tensorflow/python/autograph/operators/py_builtins.py
@@ -286,6 +286,9 @@ def _tf_dataset_len(s):
   msg = gen_string_ops.string_join(
       ["len requires dataset with definitive cardinality, got ",
        gen_string_ops.as_string(l)])
+  # TODO (yongtang): UNKNOWN is treated as an error.
+  # In case there are more UNKNOWN cases for dataset, we could
+  # use dataset.reduce() to find out the length (in an expensive way).
   with ops.control_dependencies([control_flow_ops.Assert(
       math_ops.logical_and(
           math_ops.not_equal(l, cardinality.INFINITE),
diff --git a/tensorflow/python/autograph/operators/py_builtins_test.py b/tensorflow/python/autograph/operators/py_builtins_test.py
index bfa185b52a2..83b618809db 100644
--- a/tensorflow/python/autograph/operators/py_builtins_test.py
+++ b/tensorflow/python/autograph/operators/py_builtins_test.py
@@ -125,19 +125,19 @@ class PyBuiltinsTest(test.TestCase):
 
   def test_len_dataset(self):
     dataset = dataset_ops.DatasetV2.from_tensor_slices([3, 2, 1])
-    self.assertEqual(py_builtins.len_(dataset), 3)
+    self.assertEqual(self.evaluate(py_builtins.len_(dataset)), 3)
 
     # graph mode
     @def_function.function(autograph=False)
     def test_fn():
       dataset = dataset_ops.DatasetV2.from_tensor_slices([3, 2, 1])
       return py_builtins.len_(dataset)
-    self.assertEqual(test_fn(), 3)
+    self.assertEqual(self.evaluate(test_fn()), 3)
 
   def test_len_dataset_infinite(self):
     dataset = dataset_ops.DatasetV2.range(5).repeat().batch(2)
     with self.assertRaises(errors_impl.InvalidArgumentError):
-      _ = py_builtins.len_(dataset)
+      _ = self.evaluate(py_builtins.len_(dataset))
 
     # graph mode
     @def_function.function
@@ -145,12 +145,12 @@ class PyBuiltinsTest(test.TestCase):
       dataset = dataset_ops.DatasetV2.range(5).repeat().batch(2)
       return py_builtins.len_(dataset)
     with self.assertRaises(errors_impl.InvalidArgumentError):
-      test_fn()
+      self.evaluate(test_fn())
 
   def test_len_dataset_unknown(self):
     dataset = dataset_ops.DatasetV2.range(5).filter(lambda _: True).batch(2)
     with self.assertRaises(errors_impl.InvalidArgumentError):
-      _ = py_builtins.len_(dataset)
+      _ = self.evaluate(py_builtins.len_(dataset))
 
     # graph mode
     @def_function.function(autograph=False)
@@ -158,7 +158,7 @@ class PyBuiltinsTest(test.TestCase):
       dataset = dataset_ops.DatasetV2.range(5).filter(lambda _: True).batch(2)
       return py_builtins.len_(dataset)
     with self.assertRaises(errors_impl.InvalidArgumentError):
-      test_fn()
+      self.evaluate(test_fn())
 
   def test_len_scalar(self):
     with self.assertRaises(ValueError):