diff --git a/python/ray/serve/backend_worker.py b/python/ray/serve/backend_worker.py
index 4cbc537e3..0d115eddb 100644
--- a/python/ray/serve/backend_worker.py
+++ b/python/ray/serve/backend_worker.py
@@ -1,5 +1,6 @@
 import traceback
 import inspect
+from collections.abc import Iterable
 
 import ray
 from ray import serve
@@ -195,8 +196,18 @@ class RayServeWorker:
             self.request_counter.add(batch_size)
             result_list = await call_method(*arg_list, **kwargs_list)
 
-            if (not isinstance(result_list,
-                               list)) or (len(result_list) != batch_size):
+            if not isinstance(result_list, Iterable) or isinstance(
+                    result_list, (dict, set)):
+                error_message = ("RayServe expects an ordered iterable object "
+                                 "but the worker returned a {}".format(
+                                     type(result_list)))
+                raise RayServeException(error_message)
+
+            # Normalize the result into a list type. This operation is fast
+            # in Python because it doesn't copy anything.
+            result_list = list(result_list)
+
+            if (len(result_list) != batch_size):
                 error_message = ("Worker doesn't preserve batch size. The "
                                  "input has length {} but the returned list "
                                  "has length {}. Please return a list of "
diff --git a/python/ray/serve/tests/test_backend_worker.py b/python/ray/serve/tests/test_backend_worker.py
index 53f515939..48a9128e5 100644
--- a/python/ray/serve/tests/test_backend_worker.py
+++ b/python/ray/serve/tests/test_backend_worker.py
@@ -1,6 +1,7 @@
 import asyncio
 
 import pytest
+import numpy as np
 
 import ray
 from ray import serve
@@ -9,6 +10,7 @@ from ray.serve.policy import RoundRobinPolicyQueueActor
 from ray.serve.backend_worker import create_backend_worker, wrap_to_ray_error
 from ray.serve.request_params import RequestMetadata
 from ray.serve.config import BackendConfig
+from ray.serve.exceptions import RayServeException
 
 pytestmark = pytest.mark.asyncio
 
@@ -151,6 +153,15 @@ async def test_task_runner_custom_method_batch(serve_instance):
         def b(self, _):
             return ["b-{}".format(i) for i in range(serve.context.batch_size)]
 
+        def error_different_size(self, _):
+            return [""] * (serve.context.batch_size * 2)
+
+        def error_non_iterable(self, _):
+            return 42
+
+        def return_np_array(self, _):
+            return np.array([1] * serve.context.batch_size).astype(np.int32)
+
     CONSUMER_NAME = "runner"
     PRODUCER_NAME = "producer"
 
@@ -163,10 +174,12 @@ async def test_task_runner_custom_method_batch(serve_instance):
             "max_batch_size": 10
         }, accepts_batches=True))
 
-    a_query_param = RequestMetadata(
-        PRODUCER_NAME, context.TaskContext.Python, call_method="a")
-    b_query_param = RequestMetadata(
-        PRODUCER_NAME, context.TaskContext.Python, call_method="b")
+    def make_request_param(call_method):
+        return RequestMetadata(
+            PRODUCER_NAME, context.TaskContext.Python, call_method=call_method)
+
+    a_query_param = make_request_param("a")
+    b_query_param = make_request_param("b")
 
     futures = [q.enqueue_request.remote(a_query_param) for _ in range(2)]
     futures += [q.enqueue_request.remote(b_query_param) for _ in range(2)]
@@ -175,3 +188,15 @@ async def test_task_runner_custom_method_batch(serve_instance):
 
     gathered = await asyncio.gather(*futures)
     assert set(gathered) == {"a-0", "a-1", "b-0", "b-1"}
+
+    with pytest.raises(RayServeException, match="doesn't preserve batch size"):
+        different_size = make_request_param("error_different_size")
+        await q.enqueue_request.remote(different_size)
+
+    with pytest.raises(RayServeException, match="iterable"):
+        non_iterable = make_request_param("error_non_iterable")
+        await q.enqueue_request.remote(non_iterable)
+
+    np_array = make_request_param("return_np_array")
+    result_np_value = await q.enqueue_request.remote(np_array)
+    assert isinstance(result_np_value, np.int32)
diff --git a/python/ray/serve/tests/test_util.py b/python/ray/serve/tests/test_util.py
index 62d63565c..b1c2254c6 100644
--- a/python/ray/serve/tests/test_util.py
+++ b/python/ray/serve/tests/test_util.py
@@ -1,5 +1,7 @@
 import json
 
+import numpy as np
+
 from ray.serve.utils import ServeEncoder
 
 
@@ -7,3 +9,14 @@ def test_bytes_encoder():
     data_before = {"inp": {"nest": b"bytes"}}
     data_after = {"inp": {"nest": "bytes"}}
     assert json.loads(json.dumps(data_before, cls=ServeEncoder)) == data_after
+
+
+def test_numpy_encoding():
+    data = [1, 2]
+    floats = np.array(data).astype(np.float32)
+    ints = floats.astype(np.int32)
+    uints = floats.astype(np.uint32)
+
+    assert json.loads(json.dumps(floats, cls=ServeEncoder)) == data
+    assert json.loads(json.dumps(ints, cls=ServeEncoder)) == data
+    assert json.loads(json.dumps(uints, cls=ServeEncoder)) == data
diff --git a/python/ray/serve/utils.py b/python/ray/serve/utils.py
index 511676a24..adf7d815e 100644
--- a/python/ray/serve/utils.py
+++ b/python/ray/serve/utils.py
@@ -69,6 +69,10 @@ class ServeEncoder(json.JSONEncoder):
         if isinstance(o, Exception):
             return str(o)
         if isinstance(o, np.ndarray):
+            if o.dtype.kind == "f":  # floats
+                o = o.astype(float)
+            if o.dtype.kind in {"i", "u"}:  # signed and unsigned integers.
+                o = o.astype(int)
             return o.tolist()
         return super().default(o)